Are there any wheels or rollers on your deck, like maybe on the rear of deck?
If so, check to make sure the wheels/rollers are free to roll. You really need to observe whether the wheels/rollers are rolling freely without any kind of drag or stalling out while you are operating the machine.
Any drag on one side of the deck will cause the deck to WANT TO turn sideways, but the linkages and hangers will not permit that, so it may be rising up instead.
My guess would be that a roller on the opposite rear position from the side that is rising would be the culprit.

Normally I would say to remove the access cover just ahead of the seat and confirm that you aren't missing a linkage c clip. If that doesn't make sense, post the model # from below the seat so we can see what you've got.

Wheels on the rear of the deck are free. The right wheel is on the grass, the left one is 1 1/2" off the grass, unless I lightly rest my foot on the deck, then it easily comes down to level w/ both wheels rolling on the grass.

I did remove the center plate, inspected and oiled all the linkage and pivot points. All (6?) of the arms are intact, w/keepers installed.

I'll try to get a model number.

Then I would concentrate on the suggestion made by tomplum.
It may be that a critical piece of linkage has come loose.
That piece of linkage probably does for the deck what your foot does when you push the deck down with it. :^)

With the clips in place , the next common thing would be one of the pivot links (they have a shoulder bolt through the frame- to which a lift link attcahes) has dropped out, one of the rear support rods slid out part way out and is holding in one side only, or possibly the cross shaft off the lift arm has a broken weld on one side.