There was an apparent reason, and it was mentioned specifically:
I feel ya, had a guy drop my group and swap for no apparent reason on LBP Karting... sort of rude to do, even if you apologize
1) Group 4 wasn't complete, so no reason for the host to be assigned to group 5.
2) They had a PS+ card ready to go, while group 5 needed a PS+ card. Trying to find one at the right price was turning into a hassle.
Either you skimmed, or didn't acknowledge the above as valid reasons, or maybe I am rude. Ok, let's go with rude.