I had this happen once: It was because I didn't push the filter on the spindle with enough force to snap it into place. When I pulled the filter housing to investigate, the filter fell right off the spindle. I have discovered that some filters have a tighter collar and require more force to seat. On the filter housing there is a reference mark which will be at "nine o'clock" (viewed from the front of the car) when the housing is sufficiently snug. TBW