This is probably bad luck. Because of the way skills work now, loss of skill can look rather dramatic, especially if the demon has had bad luck twice:

When you advance to a new level n, the game rolls how many weeks it will take to advance to n+1 - this can be somewhere between (roughly) one and 2*(n+1) weeks, and if you're unucky, then you reach level 4, but it will take 8 weeks to get to level 5. If such a demon loses an equivalent of 3 weeks skill, he falls back to a level from wchich the expected time to reach level 5 will be 3+8 = 11 weeks. Since the expected time to reach a level n is n weeks, 11 weeks to level 5 would be 5+4+2 weeks, and he'll drop to level 3, with 2 weeks to go to 2.

If this didn't make sense: it's not really important to understand it, and 99% of eressea players don't even know about it :-)

In the case of skills that you have a bonus on (like perception) this will look even more dramatic.