Why are my troops continuously being sent back?

Are they being sent back from zombie rallies? If so you need to ask the alliance member who started the rally. They will be the ones sending them back. Perhaps your troops are not strong enough? It is also possible the rally was cancelled if the player who started it decided to change target or if not enough people joined.

If they are coming back from gathering, or from reinforcing / stationing then its likely that the building was full when they arrived.