The olecranon bursa can become irritated and inflamed in a number of ways. Direct impact on the elbow can cause bleeding in the bursa. This can cause the bursa to become swollen and tender. Repeated strains on the elbow, such as resting your elbows on a hard surface, can also cause the bursa to become inflamed.
The olecranon bursa can also become infected. This may occur without any warning, or it may be caused by a small injury and infection of the skin over the bursa that spreads down into the bursa. In this the sack fills with pus, and the area around the bursa becomes hot, red, and very tender.