Forced Movement and Falling

Forced Movement Not Possible

Effects that force movement don't have a solid ruling on what happens when the destination space is occupied or otherwise unavailable.
Ruling: If the destination space is unavailable or the target cannot be moved due to space being occupied, they are instead knocked prone.

When does a fall process?

RAW: "If a flying creature is knocked prone, has its speed reduced to 0, or is otherwise deprived of the ability to move, the creature falls, unless it has the ability to hover or it is being held aloft by magic, such as by the fly spell."
Ruling: The timing on this is not explicit, so it could theoretically happen immediately or on the creature's next turn. In the interest of simplicity, these falls will happen as soon as the status takes effect.