I'm guessing you are just completing the circuit and there is a power surge that jumps the gap when the two connectors are almost touching. They do make anti spark
XT90 (
XT90-S?) connectors but I have never tried them. You should try connecting 8s with
XT150 connectors. Loud pop and large bright spark. Scares the crap out of you the first time you do it. LOL