You do randomly lose stats at level up; that's been a mechanic since classic Wizardry games.

As for how to mitigate it:
* Unlike classic Wizardry, the game doesn't force save after every level up, so you can just load if things go wrong.
* Classic Wizardry games had aging; in this game, there's a hidden stat called Energy, and when it gets low, stats are more likely to decrease. I believe reviving the character costs Energy, and being level drained might (though it doesn't lower your max HP); I've read that apparently leveling up gives a little Energy, and some items (when invoked) and chest traps can affect it as well. (I believe there's a trap that raises energy, just to make things more interesting.) I *think* class changing will also lower energy.

Also, the cap on stats is easily reached; if it goes down after that, it will still have a chance of going back up at future level ups.

Edit: Also, in my experience, stat losses aren't common, and they're definitely much rarer than stat increases.