Suppose you have an area completely walled off with structures and filled with units, such that no more units can enter.
Then, you drop a force field on it. What happens?
What happens... to what? To the structures, nothing. Any units pressed between the forcefield and the structures will just warp to the closest free position, but not across structures. Either that, or they will just stay where they are, unable to move.
Units that are forcefielded when there is no available space will stack on top of each other when necessary. See this video for an example.