It has to do with the fact that you’re supplying power to the mains on the ESC side of the integrated soft switch in the BMS.
Every time i charge my space cell based board i have to also turn on the receiver. This is because not only do the VESCs power up and wait for a PPM signal, they start to think that my 2.4GHz WiFi is some sort of ppm signal and it tries to interpret it as that and starts twitching the motors.
The only way to get around this without redesigning the BMS to have a soft switch AFTER the charging leads are tapped in would be to add in your own high current soft switch, such as the one on diyelectricskateboard.com, and then remove the switch from the BMS after permanently shorting the leads so that it is always on.
This behavior has sort of stopped me from looking for a BMS with an integrated power switch to use with my own packs. I’d rather start making or buying my own soft switches.