Bottle bomb question

So I brewed a imperial IPA awhile ago. It has been in bottles for about three weeks now stored at around 70 degrees. This was a five gallon batch that I primed with 3/4 cup of corn sugar.

Now I haven't had any problems at all personally. However I just found out that one of my family members that I gave a sixer to had a bottle bomb. I gave it to them just after a week in the bottle and told them how to store them for at least two more weeks.

Any ideas of what might have happened?

I have had that before...one bottle in a batch that blows.. I believe it is just an infected bottle.

What was your OG and FG of the batch? Also recipe if you can.

FG was a little off but not much. Don't have the exact numbers on me right now.


Double Your Pleasure IPA

Beer Style: India Pale Ale
Recipe Type: partial mash
Description:
Color is a deep copper red and body is rich, mouthfeel is very silky. The hop bitterness is very pleasing and the citrus aroma AND flavor of the Cascade hops is right up front. The alcohol note is not overpowering for 8.7%. Holds a nice off-white, creamy head that laces the glass to the bottom. This description is after 5 weeks in the bottle. It will continue to richen if left at room temp and at week 6 it developed a nice caramel malty sweet edge. It's an easy IPA to brew, just be patient. If you like a good IPA, you'll love this one!
Ingredients:

12 lbs light malt extract syrup
1/2 lb 120 crystal malt
1 oz chocolate malt
1 tsp irish moss
4 oz Chinook whole hops
4 oz Cascade whole hops (2 for steeping, 2 for dry hopping)
White Labs California ale yeast (WLP001
3/4 cup corn sugar (priming)
OG: 1.080 FG: 1.014
Primary Ferment: I transfered to secondary after bubble rate in air
Secondary Ferment: 2 weeks in secondary then racked to another carboy

Procedure:
I steep my grains (in grain socks) in 3 gallons cold water and raise the temp to 155 and hold there for 30 minutes.
Remove from heat, remove the grains and sparge with 2 cups of 150 degree water.
Add 6 lbs of the malt extract at this point and stir to fully dissolve. You can add it all at this step for a darker brew but it may inhibit hop extraction.
Bring the wort to a full boil and add 2 oz Chinook hops and start your 60 minute timer.
After 15 minutes, add the irish moss.
After 30 minutes, add 1 oz Chinook hops
After 40 minutes, add 1 oz Chinook hops
After 60 minutes, remove from heat and stir in remaining malt extract.
When extract is fully dissolved, add 2 oz Cascade hops and steep for 15 minutes.
Strain wort into 2 gallons cold water in you sanitized primary fermentor.
Top off to 5 1/2 gallons (I sparge cold water through my spent hops at this point to extract maximum flavor).
Cool to 75 degrees (wort chiller works wonders here).
Pitch the yeast and add fermentation lock.

Did you take two FG readings 3 days apart to make sure it was stable before you bottled?

No I didn't but between the primary and secondary it sat for almost a month before I bottled.

I would be surprised if is was from not completing fermentation though. It has only happened to one bottle and the others don't seem over carb'd

Another option would be infection. Either of the whole batch or an individual bottle.

Could be a defective bottle.

Usually an isolated incident (as opposed to a chain reaction type) is caused either by an in bottle infection (rather than an entire batch) or a flaw in the bottle.

It happens, but it's rare...I get maybe one every 2-3 years. Sometimes the bottles are weak, or you miss a spot in cleaning/rinsing/sanitizing, and that one goes.

Commercial beers have things like that happen too on occasion, usually though it's limited to on the bottling line.

I had a few batches when I bottled I noticed that my priming sugar didn't mix evenly causing bottle bombs. Changed my method to pouring the priming sugar slowly over the top of the racked beer in my bottling bucket and a slight swirl with a sanitized brew spoon. I haven't had any bottle bombs since. I used to rack on top but for some reason it didn't mix properly for me but works great for almost everybody else. That could be another possibility along with infection and bad bottles.