Autopathing straight to enemy Nexus wherever I click.

I joined a game using 5v5 team builder as Jungle Tryndamere. The loading screen took an exceptionally long time, with two people on my team never actually showing that they connected. When the game started I realized that wherever I clicked resulted in my character running directly down the middle lane toward the enemy Nexus. I could stop the movement using flash, or by pressing 'S' on the keyboard. After hanging out by the turret for a bit and getting to lvl 2, I found that I could also use my E to whirl around. Other than that, everything was toward their base.

I have never previously encountered this issue.

Needless to say this was a game breaking bug, and I don't particularly appreciate being chided by the "Promise us you'll never leave another game screen" after I left.