As it was explained to me - diesels don't start the same as petrol cars. A petrol car will start from igniting fluid off of a spark plug. Diesels start by compression, so if anything gets into the block and on top of a piston, when it comes to start there isn't as much space as before. So when you go to start and the piston travels up it can't travel the full length of the block so stops early, but the rod still needs to push up - hence it getting bent.
May be wrong but that's how it was explained to me.