No one exactly knows the reason why xenias pulse. Some speculate it's to increase water circulation, others say it's for food capture, some say it's to shield itself from bright lights..the list goes on.
As for the pulsing action, I agree with what iantoh has said about the pH and kH levels. I probably would add photoperiod as a possible reason too. You won't notice your xenia pulsing as much after lights off. pH drop at night might cause this effect too but your pH won't fluctuate alot at optimal kH levels.