To ride a horse well it is necessary to have a symmetrical body in terms of structure, muscle tone, flexibility and skills. In other words a rider needs to have dexterity. There is no truly symmetrical human being on this planet. We ALL have differences between our left and right sides. This creates a potential for unbalanced seat, especially, when it comes to the seatbones. Rider's seatbones must stay very centered, be connected to the saddle with good even muscle tone, be weighted evenly, be level and work with skill. This is quite a task to accomplish. Usually, a rider has one seatbone stable and the other one which slides easily to the side. Sometimes it is not easy to find the culprit. Here is a selfcheck list (examples are given for the right seatbone being unstable):

Shoulders are not level If you stand in front of the mirror in a usual relaxed manner (do not try to stand straight) you probably notice that one of your shoulders is lower then the other. The difference can be quite substantial. The sliding seatbone is on the opposite side from your low shoulder.
Rotational asymmetries It is easier to twist toward low shoulder. If someone calls your name right behind you over which shoulder will you turn around? Most likely you will turn around over your lower shoulder. What than means is the following (for example, your left shoulder is lower) - your left side collapses, you like to twist to the left advancing your right shoulder forward and your right seatbone is sliding off the saddle to the right.
Make pictures of you riding Pictures made from behind show a rider being off center to one side, especially, during turns. This will clearly show your unstable seatbone. Continue with our example. During circle to the left you are portrayed on the picture sliding off to the right with your hips. Your shoulders are most likely will lean toward the center of the circle to compensate the balance issue created by your seatbones. Pictures from circle to the right show your hips centered over saddle but your body is more vertical than your horse's body while on the circle, your horse appears to lean to the inside.
Stirrups are uneven Very often riders who have marked asymmetry in their seatbones positions have one stirrup shorter than another without realizing it. It happens because the side of sliding seatbone is closer to the stirrup and rider wants to shorten it. The other side is farther away from the stirrup and rider wants to lengthen it. If stirrups are even the rider feels more weight in one stirrup compare to the other.
One hip joint is more flexible than another If you know that one of your hip joints is more flexible than the other most likely this is the side you are sliding off.

These asymmetry issues are very deeply ingrained in our bodies. Improving or eliminating them is a very long process that involves developing awareness about how your body execute movements all the time you are active - from the moment you are getting up in the morning to the every day activities until it is time for bed - how you stand, sit, turn, walk, carry things, reach for things, bend, etc. All these actions affect what muscles are getting stretched and/or strengthened and what muscles stay idle. Day after day, year after year it can built into very substantial differences. Our horses feel these differences and react accordingly. Next time you are about to blame your horse for being stiff, not wanting to turn properly or driffting, look at yourself first. You may be the reason he is being "difficult".