scooting!

Depending on what they found when they expressed his anal glands - he may be irritated, sore or itchy.  Since he already had the scooting behaviour down to relieve his glands he may think that the same behaviour will help now as well.

 If he keeps doing it I would go back to the vet.  Maybe they didn't get everything....it's happened before!

Everything VanMorrison said.  HOWEVER -- did the vet examine him or just a tech?  Do you know if they expressed the glands externally or internally?  (Again, if only a tech saw him that could make a difference). 

The risk of infection can also be pretty high -- if they irritated it when they did that it can infect pretty easily, so you're right not to just shrug it off. 

Does this happen often?  You may want to bulk up his food a bit if so -- we've all been conditioned by the dog food companies to want those nice small poops -- except they don't express the glands on the way out and that can lead to problems.  By bulking up I mean adding some fiber or even some veggies with fiber that will help that.

 Also, from the "is it plugged in?" school of advice-giving... is his butt clean? Rascal will sometimes scoot if he needs a bit of "cleaning up," if you know what I mean. (And don't ask me - or the carpet - how we figured that one out! )

 Scooting can also be a sign of parasites/worms. Maybe have a stool sample checked.

That or it might be time for a food change.