When Little Joe and the box of nitroglycerin went tumbling down the hill, that would have been enough jostling to have blown up the Nitro.
Also, when the one mule load blew up, the shock wave would have set the others off as they were too close.
The nitroglycerin crate lids would have been closed with screws, not nails, since hammering nails to close the lids could have caused the nitro to explode.
When they pull the leaking bottle of nitroglycerin out of the box there's a huge hole in it. Certainly any jolt hard enough to cause that break would've caused the nitro to explode.