tumble on (something)
1. Literally, to trip on something with one's foot. I wasn't watching where I was going and tumbled on the root of a tree. The child tumbled on the steps of the house.
2. To find, discover, or happen upon something casually or by chance. I tumbled on this fantastic little café the other day. Let me know if you tumble on my keys—I haven't been able to find them.