Forum crashes can have a number of causes:
1. The forum software is hacked. <--- It happens, and it's not pretty.
2. The forum administrator installs an add-on which doesn't function properly. <--- It happens, and the forum administrator can usually fix this quite quickly by removing the add-on. (BACKUP is your friend.)
3. The forum administrator installs an add-on wrongly. <--- Yes, this happens, too. This is one of the reasons administrators BACK UP EVERYTHING before tinkering.
4. The forum administrator edits the forum software (either adding incorrect code or removing/changing correct code) badly. <--- See above.
5. The server host deletes files or database material by mistake. <--- The server tech guys will swear blind they didn't do this; ha. This has occurred; I have seen it.
6. The server goes down. <--- Oh, the pain, the pain! Sadly, this DOES happen.
7. Aliens tinker with the forum. <--- Not very likely. If there WERE aliens, why would they bother with a forum?
8. The laws of time and space and php rebel and leave everyone in the ordure. <--- I've never yet seen this happen!