Strange Problem

I have brewed 40 batches and have bottled 35 of them so far. However after a week of vacation I come back and two different batches had exploded bottles.

The temp in the house remained consistant, I used the same sugar mixture I have used in other batches to bottle with 5 oz of sugar and 1 and 1/4 cups of water all from the same 4 pound sugar bag I picked up from the HBS. What could cause these 4 bottles from 2 different batches to explode?

The first batch the grav readings were as follows

Batch #33
1.060 at brew
1.014 at bottling

Batch #35
1.085 at brew
1.016 at bottling

This has me at a loss.

Did you take any hydrometer readings during days prior to bottling? Any chance of infection?

Too much sugar or an infection. That's the only thing that will do it. How is the carbonation on the rest of the batch? It could just be an infection in certain bottles.

Haven't sampled any of the beers yet. I have a stockpile where the beer isn't usually consumed until it has been in the bottle for four months.

Yes a Hydro reading was taken for 3 days.

Same Sugar levels as the last 10 beers I have bottles as I bought three bags of 4 pounds sugar from the HBS at the same time. As I said to have a bottle explode on the 33rd batch bottled almost two months after it was bottled startled me.

I'd put a couple of bottles from those two batches CAREFULLY and put them in the fridge, in a paper bag and maybe a plastic bag as well and see if you can get them cold and then open them. If they gush, it is an infection. You're going to want to be careful with those bottles regardless.

Of the six pack where I had three bottles exploded, I left the bottles in the fridge for 24 hours then poped them this evening and they were tasty with no infection. No idea why those three bottles blew up. Hopefully thats the only time I have a bottle bomb.

1.014 and 1.016 FG sound a highish to me. Maybe they weren't finished? Bottling might have kicked up some yeast back into suspension, and the priming sugar might have roused it enough to finish off those last few points...

What yeast were you using, and was your attenuation what you expected based on the strain? What temps did they ferment at?

Perhaps those bottles had been used a few times and picked up a knock or two along the way, enough to create small weaknesses in the glass. Probably too small to see, but big enough to weaken the bottle to the point it couldn't hold the pressure. Just a guess.

If you're using New Belgium bottles...
I had a couple 6ers of Fat Tire, Blue Paddle and Mothership. When I was washing/de-labeling them, I noticed tiny flaws in the glass of a few of them.
Those with visible flaws I put on the curb and used the rest w/out incident. But I kept those cases in Rubbermaid totes with bath towels over them, JIC.
Every bottler gets a bad batch of glass from time to time.
About 18 months ago, Sam Adams recalled a huge number of cases due to a few cases breaking during normal handling.
I once tried to pry a crown off a commercial beer, only to have the whole neck come off with it. That was fun.

I had the same thing happen with the neck of a bottle of three buck Chuck. Got the corkscrew in, and when I applied leverage to get it out, the whole neck cracked off in my hands. Luckily there was no blood. It did make me wonder if Chuck was using extra cheap bottles.

The FG was right in line with what the kit called for.

Also 3 of my 4 bottle's that blew up were Fat Tire Bottles.

The 12oz Fat Tire bottles do seem very slight. They're noticeably smaller than my others...

+1 on the fat tire bottles. My last batch I bottled 6 recycled fat tires and found 2 bad bottles 1 at bottling and one when I tried to uncap.