Two bottles from two batches explode. Why?

This is a longer one...A little backstory:

I brewed a coffee stout and a "butterbeer" (https://www.homebrewtalk.com/f12/butterbeer-yes-hp-fame-433594/) back in September to give away some at Christmas. There were mostly sampler packs so some might have had a stout and an ipa, others had a cider and a blonde ale...you get the idea.

On New Years Eve I got a text from a friend saying my coffee stout had violently foamed over as soon as it was opened. Now to be fair, I had the same problem with that batch but before I gave it out I cracked the tops of every bottle once a day for a few days to make sure the carbonation was where it needed to be...in fact I just had my last one last night and there were no problems overflowing.

I just got a text at 4am from the same guy saying the butterbeer exploded during the night...exploded as in beer and glass everywhere. I'm really stumped here because while the beer I kept for me was a bit overcarbonated for my taste (i like my c02 to a minimum), there was absolutely no evidence that there would be any sort of a pressure problem.

For information's sake, I brew in Columbia SC and my family lives in Central PA (mountains). Could altitude/barometric pressure make that much of a difference? Or is it more likely that he just ended up with two weak bottles that couldn't handle pressure well?
I'm at a loss...I don't want to have to tell everyone to dump their potential time bombs down the drain, but I mostly don't want anyone getting hurt by an exploding bottle.

Any ideas?

Sorry, I'm an engineer and therefore a nerd.

The barometric pressure- I don't think this would be the problem. Your elevation is 300 ft where you are, and the highest point in PA (mount davis) is 3200 ft. That's a difference of about 30 kilo-pascals barometric pressure, or 4 psi. Normal carbonation is around 25 PSI. To me it seems like it may make a difference, but not a bottle bomb difference.

1) Force carb'd or bottle conditioned? If it's bottle conditioned, maybe you didn't stir in the corn sugar?? If it settled in the bottom, then your last few bottles could have way more sugar than everything else? I would think it would still dissolve fine; I'm just pulling at straws.

2) How were bottles sanitized? Did your family mention any sour tastes or anything with the geyser bottle (if they tried it)? My only other thought is maybe if a couple of bottles developed a wild yeast infection.

Otherwise i'm stumped... let me know if you find anything out.

EDIT: This should also only make a difference for the headspace in the bottle (assuming naturally carb'd). So for the actual internal pressure, it's negligible and shouldn't cause bottle-bombs. The Perceived carbonation may be different (i.e. upon opening, more head and will go flat faster), but not the absolute internal pressure.

Wow, I wasn't expecting calculations! Thanks for the reply!

1) They're bottle conditioned, but I don't think that's the problem. His was the only one (to my knowledge) that's done so much as foam over. Even if the last few bottles had more sugar, I have no way of knowing which ones were actually the last ones bottled at this point. It may be worth noting that butterbeer I had I poured hard into the glass (again, I like my beer almost flat) and about 1/2 - 2/3 of it was head initially. But, again, not even a geyser for any bottle to my knowledge.

2) At the time my process was to rinse out a bottle to get the majority of the yeast cake loose, rinse again, and let a bit of hydrogen peroxide (enough to cover the bottom) sit in it for 1-2 days, rinse again, then sanitize at bottling with one-step. My process now is pretty much the same, except I'm now a star-san convert.
He seemed to like the taste of the coffee stout and didn't mention anything about it being sour. The butterbeer didn't get to be tasted though.

I feel pretty certain I used old Red Hook bottles (http://www.packworld.com/sites/defa.../Images_Departments/Redhook.png?itok=952QfxFe), which don't have a lot of room in the neck for headspace. I usually fill to the brim, remove my wand, and bottle. There is a difference in headspace between that and longneck bottles.
That shouldn't be the difference between a slightly overcarbed beer and a bottle bomb though right?

If only one person experienced the problem, I'm guessing his were warmer and possibly agitated in travel to enable a bit more bottle conditioning than yours. Especially if yours have been kept refrigerated since then.

If just a gusher vs high carb level were the issue, then I'd guess you added too much priming sugar. But your fridge is set colder than his and thus more stays in solution when opening it.

Could possibly be a single bottle infection also i.e. had a bug that chewed through some non-fermentables. The good thing if that was the case would be it would be isolated to one bottle.

Nothing was refrigerated from the time it was bottled to the time it was consumed, save maybe about a week (over the course of about 3 months).

I do wonder if it could have been stored beside a heat source after I gave it to him...or if his house is exceptionally warm, maybe that's the difference (my house is at about 65F). That's a good point.

I'm hoping it's an isolated issue, but unfortunately I don't have any more of either batch to test.

The butterbeer bomb could have been caused by a flawed bottle. A tiny track or weakening in the glass could have triggered the explosion.