Hallux rigidus is an increasing stiffness of the big toe (hallux) joint in the foot to the point where the joint becomes rigid. The cause of this stiffness or rigidity is almost always osteoarthritis.
The osteoarthritis can develop for one of two reasons. One is that here is a progressive wear and tear of the joint over a long period of time from poor foot biomechanics causing the joint to move in a slightly abnormal way. With time, this dysfunction damages the cartilage in the joint that wears down to cause the osteoarthritis. This causes progressive pain in the joint and the restricted range of motion.
The other reason could be that there is a one off incident of trauma to the joint (or a few incidents) that causes some damage in the joint. This too can lead to the development of osteoarthritis over time.