i am making some analyses via text on a forum, so hopefully some or all of this helpful.

1. video yourself -> take a deep breath in and let it out. what moved? did you take in air via the chest? shoulders (ie. they rose up)? belly?

likely you took air in via shoulders and/or chest. poor breathing patterns can overwork the traps. if you are constantly using the clavicular muscles to take in air, you are using the traps for breathing 1000s of times a day. this will really wipe them out, make them sore and tight.

this can cause them to be overactive, and may make them contract first for any overhead movement. this is bad on a variety of levels.

i would look at learning how to breath diaphragmatically to take the stress off the traps for breathing.

2. it is possible that now that your traps are activating or over activating, that the opposite muscles to their action are weak or inactive. this would be the lats mostly, maybe likely the external rotators of the shoulder.  

when i was taught kettlebells, i was taught to press overhead with a packed shoulder. what this meant was to be able to keep the shoulder joint in its socket while pressing upward. it takes some practice and will need the activation of the muscles like the lats and external rotators to keep the shoulder lower while you press overhead.

3. you should also look into changing the way your muscles fire as you press overhead. this is related to 2. you need to train yourself to raise your arms overhead without just (over)firing the traps.  one good one i found in Sue Falsone’s DVD The Shoulder where you hold a theraband with one hand, wrap it around under the armpits from one side to the other and back to your other hand.  then you attempt to raise your arms to about 45 deg without shrugging up, which will cause the theraband to lift from the armpit location. the theraband gives you feedback on whether or not you are rotating the arms with or without trap action.

4. it is possible that other exercises or movements caused this problem. for example, shrugging with traditional weight lifting could also exacerbate this problem. if you are still in the weight room doing shrugs, i would lay off them for the time being. i believe the snatch has an initial shrug so you may want to stop that for now until you balance out the muscles, and are able to move your arms overhead without overactive trap action.