My opponent made too many scute swarm tokens, and the timer system booted me.

While resolving a great multitude of scute swarm triggers, my opponents timer rope began to run out, followed by a time out being used. After that was exhausted, my timer rope began to burn down, despite having no possible actions available to me. I had one untapped land (Hedge Maze), and one untapped creature with a mana ability, but it was summoning sick. Also no cards in hand that were available, due to the lack on available mana.

So while I had no actions available to me, was automatically set to auto-pass, and while Arena was resolving my opponent's triggers, my timer was depleted.

When my next turn started, the game rushed me through my draw step and right to my end step triggers of Chimil, the Inner Sun, then selecting nothing to cast, and passing to my opponent's next turn. During that turn they triggered Scute Swarm again, and the game made me auto-forfeit due to running out of time.

Also, the gear symbol to open up the menu disappeared from the top-right corner of the screen during the first turn mentioned, so conceding the game was no longer an option.