Jack and David died under tragic circumstances, David in particular could've been avoided given his dad told him to make his dates in Japan despite feeling sick. Mike and Chris wanted to be with their brothers and felt constant pressure from being below the standard of David, Kerry and Kevin. A big combination of pressure and endless tragedy. If you're Kerry Von Erich and you've lived through like four siblings dying at such a young age, you'd probably go nuts.
The show "Dark Side of the Ring" covered this in detail.