Noticed that behaviour too, with a pack of Launcher Cyclopses. I shot one, and a great pack startet to move east. Also must have happened before too, because I just had killed the whole bunch - and they were grouped in the east of their spot before.
Only when I moved closer they came after me. And thats not really easy to do with all mobs.

Also regarding the same topic:
In the northern seaside Sectors the enemies move north. always to the north. And sooner or later they walk out in the water and then are bobbing up and down there when you shoot at them. and if you have the wrong weapon (Fusion Rifle for example) you can't hit them when they are underwater. Laser is fine for it if i remember right. But the mobs stop moving out there and stop being a threat more or less.