Possibly they were expecting an eschatological event involving the Temple? 

That’s my guess.  They went to Jerusalem during Passover in the first place expecting an eschatological event and returned because they expected an imminent Parousia.  On the other hand it makes more sense to imagine that the “appearances” took place in Galilee.  Notice how Mark separates the Empty Tomb experience from any potential appearances.  And in Paul’s I Cor 15 credo why would James see Jesus in Jerusalem if he wasn’t already a believer? And under what scenario would you have 500 brothers present to see the resurrected Jesus in Jerusalem? 

So here’s my historical scenario.  Jesus and his immediate disciples, perhaps accompanied by an indeterminate group of fellow travelers, made the journey to Jerusalem during Passover expecting the Parousia.  There was some kind of incident in the Temple but instead of vindication by God, Jesus was immediately arrested, condemned and crucified.  (It follows from this scenario that the stories of the trials before the Sanhedrin and Pilate are non-historical.)  The disciples escaped to Galilee where after some indeterminate interval people in the community began having experiences which they interpreted as the beginning of the Parousia, heralded by appearances of Jesus.  They returned to Jerusalem for the same reason they went the first time.  A small colony remained in Jerusalem until the Revolt while a larger Jesus movement sprang up in Galilee.   

If he subsequently appeared to them in Galilee, there was no reason for them to return to Jerusalem.

Could they not have experienced Parousia in Galilee?

Likely they returned for the same reason they went the first time.  They saw some kind of eschatological association between the Temple and the Parousia, perhaps?  Galilee may have been a backwater even in the world of the spirit.

When you say “some indeterminate interval”, do you mean a few days, weeks, months or even a year or two?

How it actually worked we don’t know.  Historically I mean.  The NT makes it seem like a hasty phenomenon, unfolding within a few days of the crucifixion and wrapping up rather quickly as well. I suspect this tradition began as a way of distinguishing “official” resurrection appearances and to validate the early leadership.  Note that one of Paul’s difficulties with his opponents was his claim to apostleship since he wasn’t one of the original disciples.