I suspect that the enemies’ pathing is one of the first things to go when the CPU is under too heavy a load. In a recent game which I’d played for a long time (I didn’t have a lot of stuff, but I was noticing slowdown due to both graphical issues and errors), I had goblin camps simply up and leave because they couldn’t find their way to attack the gates in my stone fences.
While this would be a reasonable decision if it’s the case (after all, having lots of enemies simulated while the game is on the edge of breaking down would mean it’s very hard to do anything about them; so making enemies be the first thing to start failing means that the town can be saved if the player manages to get their memory usage back under control); it’s also a great opportunity to give the player a warning that their CPU is being over-taxed.
One such option is to have the enemies send you a message, along the lines of “[random enemy in the group] say there big bugs in your town. We no like bugs, we going now before we get bugs too!”, and then have the game gracefully despawn those enemies to save some CPU load.
That would give the player a warning that they’re over-doing something, but it would do it in a rather amusing way. The message could have an attached explanation about what’s really going on, as well as suggestions for how to improve memory usage (and even a reminder/request to upload the save for Radiant to diagnose if the memory issue isn’t resolved by those steps).