​There are various reasons that can lead to the development of this condition. Subacromial impingement could be due to:

1. Muscle imbalance. This refers to the tone or strength of the muscle around a joint and may be caused by unbalanced training regimes or poor technique.
2. Mechanical reasons. Bone spur formation on the surface of the acromion, reducing the subacromial space and increasing the risk of impingement.
3. Direct injury to the tendon.