Frequent flexing of the knee joint and pressure upon it can cause knee pain due to inflammation of the tendons or the friction of bone on bone in poorly lubricated interfaces.
A baker's cyst is a hernia of the synovial sac that encloses the knee joint. Since the knee joint is so large, and carries the weight of the body with every step, this is the joint where where pressure most commonly causes the joint fluid to form a hernia. The symptoms of a baker's cyst are directly related to where the hernia happens. Nearly all of these occur on the back of the knee because that is where the protecting tissues are the thinnest. This part of the knee has the greatest concentration of nerves and the pressure of the cyst on a nerve can cause extreme knee and leg pain.
The surgical approach to a baker's cyst to approach where it is coming out of the ruptured sac, almost always from the rear. The idea is to remove the cyst without breaking it and then suture up the weakened part of the sac that allowed the tissue to bulge in the first instance.