Ka24de detonation marks on pistons

[240trooper]

So long story short my car was smoking and my friend and I thought it was the valve seals. He went to change them and dropped a valve in the cylinder. While trying to get it out he bent it and had to take the head off. When he did he saw detonation marks on all four pistons BUT ONLY ON THE INTAKE SIDES. The no. 1 piston was by far the worst and 3-4 weren't too bad but of course we are going to change them any. Luckily I have everything to fix the problem but I want to find out what caused it before I start driving it again. Just glad we found the problem before it got worse.

The motor:
Rebuilt it 3 years ago with super tech pistons, arp head studs, cosworth hg, all new gaskets, polished crank, bored cylinders .20, enthalpy tune ( z32 maf, t28, 91 octane, 255 lph pump, sr injectors), boost was only at 10 psi

I'm sure I am forgetting something. My friend said he saw the same problem on an sr before but couldn't remember what caused it. If there is anyone who can help i would greatly appreciate it. Thanks!