Two rare posibilities;
-your Y harnes is really a reversing Y harnes. made to do just what its doing to drive two elevator servos the same way.
- you've gotten a very rare wired the wrong way servo. In the old days of RC you could get "reversed" servos. Don't know if this standard has faided out.

Those are the only two explanations for what you have.

Try one different servo and see if anything changes. If not swap out the Y harnes