tacked up horse sitting down in aisle

That is so odd!  (at least to me it sounds odd).  I guess my first reaction would be to make him work when he attempts that so he associates that behavior with having to work (backing up, circling, etc)  I'd LOVE to know how he learned that trick!

That does sound interesting!  What do his eyes look like when he does that?  If his ears are back and his eyes are wide, he's basically just freaking out.  Horses are naturally claustrophobic animals, and knowing ( or even just thinking )that he can't move could cause him to not be able to move, which may be why he's sitting down and not running away - he goes internal, introverted.  He can't move like he wants to.  If that's the case, I would suggest saddling in a more open environment and cinching up slowly (i.e. cinch up a little, move him around and let him breathe, then cinch up a little more, etc.)  It may be you're just going too fast and it scares him.  Make sure he's not tense at all before you cinch though or he's going to feel trapped and if you push him to far, he may explode which would be dangerous for both of you!  We don't want that!  Let me know. . . . . .

That is pretty strange but I have heard of horses being so girth sensitive that they will completely set back and sit down. I would recommend possibly have someone hold him when you tack up and then tack him up and then hop up on him. Try to cinch him up slowly. Also I would try putting the bridal on first before putting the saddle on. Ive heard of this happening before. Try not to let him sit down because the more he does this the harder its gonna get to fix. Good Luck!

All good thoughts and I will just add another for you to rule out as a cause. 
 
The horse is possibly in pain.  It may need to be seen by a chiro to be sure this is not an alignment issue.  Its so easy for a horse to hurt themselves with strains, sprains, kicks, or falls that you aren't aware of.  I had a horse get in a trailer in good shape and get off with a rib over another.  Not sure how she managed that, but it took a visit to the chiro to adjust her rib and some vertebrae in her neck.
 
Good luck.