At about 10:15 a.m., a garbage collector had left the truck on Cristy Way near Joseph Drive to collect a cart when the truck rolled down a hill, spokeswoman Karen Stern said.
The truck pushed a parked vehicle into a home before crashing into a second home, Stern said. No injuries were reported.
"This is highly unusual," she said. "We're all interested in determining what happened."
She said safety is a priority at Waste Management, and noted that drivers start their days with a checklist to assess the mechanical safety of their vehicles.
Stern said the driver involved has been with Waste Management for at least 10 years.
Members of the Alameda County Sheriff's Office and the California Highway Patrol also responded to the scene.