If there is a thick coat of resin, or a deep, wide hole you are patching, it goes off quicker too, and it can get warm from the chemical reaction going on. Okay, think of the liquid resin as a whole lot of short pieces of chain. If you link them together, the stuff hardens. A hot, fast setting batch will only get a chance to make a few connections, tie together a few pieces of the chain. It is kinda weak compared to the long chains that are made up in a slow batch.
Hot batches tend to crack.
They are weaker in any case. And of course, hot batches are literally hot. The chemical reaction happens fast and ugly things can happen. I once watched a board catch on fire. Not one of my boards, fortunately.