I never heard of priming with turbinado sugar....maybe it's causing a little extra yeast activity? Also, what temp are you conditioning your bottles at? Anything over 80 will put you in the danger zone, and you gotta watch for that this summer.
I'm having the same problem with an imperial that I bottled over 10 months ago - but it's on account not attenuating fully - in your case, your beer fermented out just fine, and I'm suspecting it's a combination of the type of sugar you used with warmer than normal temps. Get those bottles cooled down, maybe a fridge, or simply next to an A/C vent, and be really careful with them.....I've seen glass spread all over a room from exploding bottles, along with holes on the shelves they were sitting on, so proceed w/caution! - in fact, I'm not going to touch my current batch of imperials until the temps start cooling.