[GAMEPLAY] I can't leave the spawn area

In my last game, I played Tristana. I could not leave the circular spawn area after the initial 15 seconds lockdown.

I kept trying to walk out. I tried recalling. Using Flash took me to the bottom of the steps, but after I tried to move, Tristana moved back up to circular spawn area. Eventually my teammates accumulated enough xp for me to get Rocket Jump. Again, I could jump to the bottom of the steps, but any movement after that forced me to walk back to the spawn area.