I experience this many times often when I have sat down for long hours. This is because blood circulation throughout the body is limited in that part of the body. When you stand up, blood tend to flow at a faster rate almost the rate of your heartbeat causing pulsating buttocks. Also, this might be due to stress especially when you're experiencing something for the first time like an interview or presentation. Blood in the buttock tends to flow faster competing with heart beats.
This can be reduced by changing your sitting positions often or taking several breaks from sitting down. Also when doing presentation for the first time, its advisable to remain calm no matter what. Pulsating buttocks isn't a serious problem you can handle it.