It's the switch internals, perhaps in combination with the plate/case construction.
I've had a single switch ping here and there (e.g., a couple of blacks on SteelSeries 6Gv2), but almost every keyboard with clears was said to ping a few years back IIRC.
Damping foam inside the case definitely can help.
If there's one type of switch, that pings like crazy, though, it's springs in a barrel plate: buckling springs or other.
Oh, and Pingmaster.