Basically, the original 68000 isn't capable of atomically restarting an instruction interrupted during a memory access cycle, and so there's no way to implement a standard MMU.
So Apollo just chucked in a slave CPU that would detect the interruption of the master CPU, halt it, deal with any remapping or what have you, and then just completely reset the master CPU.
On that regard, it's a shame the Amiga released (1985) with a 68000 cpu. Particularly, move from SR became privileged 010+ and thus caused problems when Amiga finally moved past 68000. Those could have been easily avoided by releasing Amiga on 68010 to begin with, which is also a CPU with slightly higher IPC.
They compounded the issue by using 68000 again on A500/A2000 (1987), for negligible savings.
Of course, this does pale next to the gross mismanagement Commodore did of the Amiga thereon, which ultimately led to Commodore's own demise.