I have had 2 brews and a mate, one brew recently that have turned nuclear. And it has been on or about day 20-21 after bottling in all instances. All brews were a different style but the same manufacturer, namely Mangrove Jack. Were we live is currently getting warm, 33-35C. I’ve saved most of my brews by either putting them on ice or in the fridge. They taste perfect. When we were discussing the possibilities we speculated about the yeast potentially reactivating on or around the 3 week mark, exacerbated by the high temps. We have both made MJ brews a few months ago when it was cooler with no problems. We brew slightly differently, he goes for lower alcohol and I for the recommend recipe. We haven’t encountered the same problem with Black Rock or Morgan’s that we also use. Look forward to your input. Waz
OK. You're brewing kit beers, so I'm assuming your wort composition has been more or less consistent (i.e. 1 tin of hopped malt extract, 1 kg of brew blend which is mostly dextrose with some maltodextrin and some dry malt extract blended in, and water up to 23 litres / 6 gallons. That means your bottle bombs could only have a few possible causes.
You bottled too early. Did you get the same final gravity (typically 1.010 for MJ kit beers) every time? If not, a high FG means you're bottling too early and too much fermentation continues in the bottle.
Contamination with a wild yeast that ferments sugars (typically polysaccharides and dextrins) that the regular MJ kit yeast leaves alone. Do your explosive beers taste dryer (with less body) than the non-explosive ones?
Overfilling the bottles. You need enough headspace under the cap so that the air can compress sufficiently when CO2 develops.
You say the beer tastes fine, so I'm ruling out bacterial infection on the basis of that.
If my assumption of consistent wort composition is wrong and you have varied your ingredients, there could be a complex sugar in the mix that ferments out very slowly, so that too much fermentation is delayed until after bottling. Also note that some yeast strains struggle with some malt extracts, so some yeast/extract combinations can also lead to overcarbonated (although seldom explosive) beers. If you have varied the yeasts you use rather than use the MJ kit yeast for all beers, this might also be a factor.
I believe the issue was a combination of post bottling temperature and hop creep. Whilst not excessively hopping(20-30g) dry, it’s hot in the tropics. My shed could get to 32-34C during the day. Backed off the priming sugar bit by bit, nah. Time to keg. Another challenge but not as potential messy. The Kveik yeast works really well here, no fridge needed and over in 2-3 days. Yes just using extract cans, hot storage shortens there shelf life as I’ve learned. All the best for 2021 and a new Pres.