exactly, if your target was say Liverpool you'd have a number of alternative targets in case Liverpool was obscured (probably Manchester, Crewe, etc) - if they were all obscured they they're would be a chance you'd have returning bombers dumping them on the way back, but its just as likely they'd just drop them anywhere rather than travel back across England waiting for Sunderland - and why Sunderland, why not Newcastle, Hartlepool, Middlesbrough, Hull depending on your route back
Plus i also mentioned upthread that the majority of the bombers were based in France, Belgium, Holland & Denmark so if we're on a return flightpath to those locations then its probable that Scotland was the target, in which case they'd be more likely to try and bomb a Scottish port on the way out (why travel back over England when you can nip out into the relative safety of the North Sea for your trip home)
Sunderland was targeted because of the shipyards, it was'nt bombed by accident