I have had my drivers seat do something similar when i have moved it from the wifes driving position, it'll feel fine then will shift slightly and 'click' into position if you brake or accelerate heavily, felt like it was only locked in on one side if that makes sense? Doesnt move after that and has only happened a few times to be fair..
I lubed up any obvious latches and the mechanism under the seat with some Quantum maintenance spray and hasnt done it since