How to fix raised shoulder posture?
In my experience it may be a lack of core strength.  So you end up leaning on your arms more, to try and lighten the load on your mid-torso muscles and lower back.

Try moving around more at work, a standing desk might help.  Some stretches throughout your work day and before riding too.

If you don't wear cycling pants, consider getting some.  They offer support around the waist which I found helpful.