That's on purpose, and well noticed. There are actually two things that happened here:
- Dodge reduction
If you have more than two monsters attacking you at the same time, your chance to dodge an attack is reduced. Even if you can dodge a level 20 monster with a 95% chance at level 30, if you have ten monsters meleeing you, your chance to dodge them will be much lower (if you want, I can check what exactly it is, but you'll be hit quite a lot). This doesn't necessarily mean they will crit; most of the attacks that land will still be regular hits.
- Soft Enrage
If you do not deal any damage at all to a monster for five minutes, it will begin landing a critical hit on every attack, until it resets, or dies (critical hits pierce through defense and armor, which is why the number's higher). What this tries to prevent is someone collecting all the mobs on the map (as a high level character), and then going away for a long time, making the map completely safe for any low level characters, and possibly also depriving them of monsters.
I completely agree that it's a really subtle effect (since there is no explanation for what's happening, they simply begin landing crits), and I will try to make it more obvious (probably giving them a buff).
(By the way, even if you see a temporary problem, report it. I can likely check what happened, and what was going on. A problem that fixes itself over time is still there, just waiting to occur again.)
