the nut too tight. the technician shouldn't tighten it as it will not have any free space and it will wobble. normally just screw it until pass the hole and insert a secure clip as to not let the screw being screwed out in the long run. many unexperience technician will screw it tight and did not put a safety clip/pin which comes together with the fan. there is also another safety wire which can hold the fan in case if it drops down. touch wood will not happen

do not tighten the nuts which hold the U on the ceiling. it is meant to be loose and it has a safety clip\pin to prevent the loose not from coming out.
when you tighten it, the fan will get wobbly as what you are experiencing now