An agitator is something or someone that or who agitates in one or the other of the senses of the word agitate.
Considering first the non-living agitators, one example is an agitator in a washing machine. This gadget swishes around the clothes, presumably so that the soap and water interact more vigorously with the stains on the clothes, thereby helping to clean off the clothes.
I suspect the fact that marketers can make an agitator seem like a very important and impressive thing to have in a washing machine and, therefore, worth paying for also has something to do with it. But that may just be the cynic in me.
People who are agitators also stir things up, but in this case it is emotions that are stirred up. This sort of agitator generally tries to elicit a not-altogether benign reaction out of people in the pursuit of some cause.
In contrast, people who try to stir up feelings in a positive way are generally not called agitators. For example, cheerleaders at sporting events aren’t usually referred to as agitators.
Then again, busty cheerleaders who display a lot of cleavage as they bounce around on the field may agitate a certain segment of the population. But that’s something completely different.