There are so many possible causes of muscle tightness that it would take a very long time to cover them all. And the trapezius is more susceptible to tightness due to being involved in so many movements and stabilizations of the upper body.
A very well trained fitness professional or Physical Therapist can often determine some of the potential causes fairly quickly. But often as one issue is reconciled, others are uncovered. Usually having a tightness in one area leads to slack in opposing muscles and tightness in compensating muscles and so on.
In general, effective treatment of tight muscles involves reconditioning the tissues in the affected areas first. This is usually done with myofascial release techniques, massage, and other physical therapy treatments. When the tissues are less excitable, they can begin to be gently stretched to help reposition the joints affected into proper alignment. And then the muscles involved can begin to be restrengthened.
A great many injuries and other issues are the result of not following the proper progressions for improving fitness. Many people want to get to the heavy lifting as quickly as they can or to running sooner than they should, etc. Short cuts and impatience is a recipe for injury and poor results. Or people find themselves in need of performing a task, like shoveling snow, for which they are not fit enough or have not performed similar motions under loaded condition in a long time. This could also be avoid with a little careful consideration. In the example of shoveling snow, the person could use a smaller shovel initially or not try to move as much snow with a bigger shovel for the first three or four times that they need to shovel snow. Of course, it would be wise to be conservative whenever you are doing a strenuous chore, but many times people are just impatient.
I teach CECs and Fitness Instructor courses on the Big Island of Hawaii. Anyone interested in checking out my offerings can go to my website – www.hawaiifitnessacademy.com . And if you plan to be on the Big Island sometime, shoot me an email. I enjoy meeting other Fitness Professionals and spreading Aloha in the fitness community.