Hi MK. There are (can be) several reasons for a tight trapezius muscle. One common cause is stress which can manifest itself as tight neck and trapezius muscles.
Poor posture, or poor alignment with the head protracting
For example hyperlordosis will increase the cyphosis and the forward flexion of the head as a compensatory pattern and will put stress on the upper trapezius who has then to carry a heavy weight (the head) and also poor midd and lower trapezius strength and latissimus strength (unbalanced strength in back muscles and pectoralis major). All the muscles that lower the shoulders should be reinforced but before we need to stretch the tight neck extensors and mobilize the spine.