Understanding source of EMI
19.2Mhz is not used directly inside the chip - it's instead feed into internal PLL which then generate all the frequencies needed.

So ether 3.2Mhz is used internally to drive large capacitive loads (unlikely, too low for anything useful in USB3.0 chip), or it's just some resonance on the PCB or decoupling capacitors.

For further investigation I would suggest to monitor if these peaks move when you heat capacitors and crystal separately to 100C.