Bottle-Bombs from Infection?

No bottle bombs yet but I bottled a dry irish stout two days before new years, been sat in food-grade plastic buckets for a shade over three weeks. Gravity was the same for the last week but a pellicle formed over both buckets in the last week, but hey its an irish stout so it should have a sour bite right? OG was 1.051 and it finished up at 1.022. I mashed high and figured given the 2# carahell and nearly 3# roasted malts this was probably not a stuck fermentation but as far as it was going to go. Two weeks after bottling I tried one which was great but a touch sweet, still needing time to condition so a week later I tried one, perfect. Week later I try one and it goes off like a rocket when I pop the cap, same again with another one. Obviously some sugars are getting munched up by something. Will this just be the yeast and it should be as carbed as it will get or could it be the infection which will continue to eat until there's nothing left or it ends up painting the inside of my spare/conditioning room with my beer? I'm going to get this in the fridge after I clean it tomorrow but I'm more interested to know what kicked fermentation off again since I've not had this problem before after 100+ batches.

Cheers

It is most likely the yeast. Were it an infection it would obviously smell bad. Also that infection would have a difficult time as the yeast has already made it hostile with alcohol.

It is quite likely the yeast simply were not finished. Your excess carbonation came from the left over sugar and/or any excess bottling sugar you gave it.

Ill be getting them in the fridge shortly, any ideas how to release some of the pressure? If heard of chilling them as much as possible then cracking the lid and resealing but I doubt that would get an airtight seal?

You would want to use new caps.

cant see that being an option. I get about a second before the foam erupts. I might give one a try after a couple of weeks in the fridge, if it doesn't work I can always drink it

I may be way out in left field, but I seem to get a similar result if my bier fridge temp is set too low.