Adding tailwind component would initialy bring reduction in IAS/Mach, no? Only when the aircraft is "established" in the new layer of air it will deviate above (forward of) calculated descent trajectory. The increase in speed is a result of AP guidance attempting to regain VPTH if that is the active mode. Of course it should stay clear of the barber pole. My bet is on temperature change. The existence of air layer with different properties is exhibited by the sudden and significant wind change. Again, positive tailwind shear makes your airspeed drop, not the other way around.
FD (the un-real)