Actually, you can see the lead P-51 pilot touches down before the chase pilot is able to, thus slowing down into the P-51a (A36 as you call it, I was friends with it's builder, the man that died in the crash). The -51a pilot pulls up to avoid chopping the cockpit of the lead airplane to shreds, and throughout the process the propellor broke off the shaft. A combination stall/torque flips the chase over.  He saved the life of that lead pilot.