What is causing a loud static POP when PTT mic is keyed?
The reason why a series switch causes a POP to happen when the PTT is depressed but a shunt switch does not has to do with the DC behavor of a dynamic microphone, combined with charge buildup on the input AC coupling capacitors of the microphone stage.

There is a non-zero current flowing at each input to the microphone amplifier -- this is an input bias current, and is present in all stages that use solid-state inputs. To prevent this bias current from flowing through the mic, and to prevent phantom power from driving the inputs outside the input stage common mode voltage range when dealing with phantom powered condenser microphones, just about all microphone amplifier stages have front-end AC coupling/DC block capacitors.  These capacitors charge up to a significant voltage when an open circuit to DC is present (such as a condenser mic, a dynamic mic with a series PTT switch in the open position, or merely nothing at all).

However, dynamic mics, unlike their condenser counterparts, have a very low DC resistance (just the coil of wire).  This means that when the PTT switch on your offending microphone is closed, the capacitors can now discharge, forming a transient that creates the POP you are hearing -- the decrease in loudness as you keep using the PTT switch is because the capacitors aren't fully charging before being discharged again.

The reason microphones with a shunt PTT avoid this is because that shunt PTT switch shorts the microphone out when in the OFF position, keeping the capacitors from charging up to begin with.