This game was aborted on move away from checkmate... How?

en.lichess.org/RfaG5WX9
I was one move away from delivering checkmate when my opponent stopped playing and started offering draw.
I browsed other tabs in the meanwhile, and when I came back I saw "game aborted"

This happens when Lichess is being updated - because the site may become unstable for a while. It's likely that your opponent was trying to play but the moves weren't getting registered by the server. In the game there are automated messages saying that the site is being updated, and if the instability is bad, any games in progress are aborted automatically.

Your opponent must have crashed the site when facing mate in one.
Aux grands maux, les grands remèdes.
:-)