Had this a few times recently. Finish a ride and the Garmin presents me with a crazy new FTP that isn't reflected by the ride data, along with an updated 'Best Power (20 Minutes)' Personal Record.
Can't see any particular pattern. Any fixes? Doesn't seem to do much damage to Garmin Connect beyond being a nuisance at the end of a ride (assuming you press 'skip' and not 'update'!!)