who gets foul, if any

[wyo96]

JV Boys last night.

B1 has inside rebounding position on A1. On the rebound both go straight up within their vertical plane. A2 also jumps for the rebound and crashes into the back of A1 with enough force to displace A1 into B1 and displace B1.

I whistle, prelim a push on A2 and report it. The coach says that A2 hit A1, how is that a foul?

A2 clearly "caused" the foul, but A1 made the contact with B1. I thought the contact caused significant displacement, so I needed a foul, but with hindsight is this a no call? Should this be a foul on A1?

Looking back, I believe I needed to call the foul on A1, even though he didn't do anything wrong. Correct? Is there a rule reference/case book to support this or is it as simple as "he's the guy who displaced B2"

Thanks in advance.

ps. you will notice that I don't post much, but I read this forum every day.