Corrected entry: When Katniss destroys the supplies and the mines are going off, one explosion comes from directly underneath the pile. Even if the Careers had the sense to make a hollow cavity keeping it from touching the gear, there's absolutely no reason they would place all their supplies on top of a mine, considering they moved all the others. It can't have been intended purely as a trap on that scale, otherwise when the careers return they'd be happy the trap worked, not furious that all their supplies were destroyed.
Brainiacazoid
4th Apr 2012
Correction: They were angry that the supplies were destroyed for nothing, since the trap was tripped without killing anyone. They were willing to sacrifice some supplies if it meant the death of another tribute. But this was just a waste, and showed they'd been outsmarted. I'd be angry too.
Phixius ★
In the book, it is explained that normally only one mine would explode, leaving the supplies mostly intact. Katniss shoots the apple bag to trigger several at once, which sets off the others. The same would happen if a whole group of tributes approached the pile. A big enough group could manage to defeat the careers or steal the supplies, so having everything blow up in that case seems a rational choice.