A couple of couplers
I used to run JD935s. This happened several times. As to WHY this happened, considering the amount if time in storage, I'd say dry rot got to it. No worries though, simple to replace and should last a long time for the new owner. There is no ryme or reason in trying to align the new one. It is supposed to be cattywhompus. It flexes with the motor since it is on rubber mounts and the hydro is mounted solid. As you said, be sure the bolts are tight. Good luck.
We're GROWING to be the best!