The dumpster is in a locked room, which means that the cat could only have been dropped via the building's garbage chute. The garbage chutes are accessible to residents through doors, so it's unlikely that the cat went to the chute on its own and fell in.
Hawks Ridge's staff have given the cat's owner much more credit than I would have:
The maintenance supervisor has no clue if the cat was thrown in on purpose, or by accident. “I don't know, you know? Could have been in a box that somebody threw out,” said Warson. “He was sleeping, and they didn't know it. It's hard to tell how it happened."The cat was eventually taken to the Iowa City Animal Care and Adoption Center where it will eventually be adopted by somebody who won't drop it down a garbage chute.