That's what I was thinking too, until
@peachykeen pointed out, and I remembered, that the tow tractor switch and spur is actually located at the south end of the spur from the express beam to the Epcot beam, so any monorail train stopped at exactly that point would indeed block tractors from either location. Even before that switch was installed, tractors could also have been blocked by a train stuck on the switch to the shop spur near the Magic Kingdom.
If I recall correctly, Disneyland (at least) once had a similar problem with a disabled monorail stuck on the shop switch, and after evacuating the train, had to tow it off the switch from the ground.