Generally it is people finding more bugs and assigning them to you.
Monday/Tuesday I spent fixing AI pathing freezing in skyscrapers and optimizing the CPU load for those buildings, which is like a worse case pathing situation with around 200k path nodes. In the process I noticed light level issues where zombies can't see you half the time. Digging into that today I found some problems in that code. Meanwhile 3 bugs were assigned to me so far this week.
Like digging a hole, but people keep tossing dirt back in.