I have a grinder consisting of five 7x7 sections of Dreadful Dirt, with fans blowing all the spawns into one Masher. Normally this works just fine as long as I stick around.
Whenever I return to the area after it has been unloaded for a bit, I can only assume that it tries to spawn every mob that should have spawned while I was away, all at once.

This is a flood of so many mobs that the fans stop working. They're still on and spinning, but the mobs don't care. Some mobs just stand in midair, defying gravity as my client immediately runs out of memory.

I'm using a multiplayer ad-hoc LAN server if that makes a difference, I don't think this happened while I built it in singleplayer.

While it isn't a fix to the problem, the simplest solution would simply be to chunk load this area. This can be done via vanilla methods (the nether portal trick) or through a chunk loading mod such as FTBUtilities and its addons. This shouldn't have a strongly negative effect on the server, but will maintain so long as the chunks are loaded.