Duh. You live at the bottom of a hill then you can expect what’s up hill must come down. Where is came from is easy - up the hill. Why it came down is also easy - erosion and gravity.
I used to live up the hill from there in Ramona back in my Navy days. I remember coming around a bend on Hwy 67 early one foggy morning and finding a 12 foot diameter boulder sitting in the right lane. Woke me right the heck up. I think it had to be dynamited and hauled off in multiple dump trucks.
“All of a sudden this boulder started coming down, but no cause, I have no idea why that rock would have come down today.”
Why is this a surprise. Poway is full of huge boulders. Many homes are nearly surrounded by them. Why is this even a story considering the area?
Erosion Always Wins.
Now that’s what you call real rock n roll.