disappear I  was riding and the saddle when I got home was sitting on his withers. He just turned 18 this year. 

Yes. Horses are like humans. As they age their muscles deteriorate. 

At 18 my gelding started losing top line, he also started randomly kicking out when asked to move out which was very unlike him and when running barrels he would run hard between barrels and then felt like he stood himself up and very stiffly got himself around the turn, again, very unlilke his running style. I talked to my farrier about it, we checked and double checked shoeing and making sure his heel hit first, angles were balanced, etc.  Got to really looking at him and saw that his top line had really sunk, tried trotting cavelettis...finally one day after he kicked out when I merely went to ask him to walk forwad and I got after him, his first reaction was to double down on the kick out. That is when I knew something was really not right, this horse has never ever miss behaved.  I went back to the trailer, pulled his saddle and and pad, put his saddle back on with no pad and checked it. It was bridging badly!  

after trying a lot of different pads, I did end up getting a treeless saddle to accomadate his new topline issues.  And after about a year of riding in the treeless saddle a lot of his topline actually came back. I think the saddle had been an issue long before he acted out and what I thought was him just losing a step due to his age was the saddle not fitting and him making adjustments to how he ran and turned to try to keep out of pain, which then escalated the loss in toplline.