Human nature.
I try to think about stops like this imaginary scenaro:
Your back in college, the professor writes your trade (entry) on the board ...
Then he pops a quiz to his 100 students and asks them: "where do you put your stop?"
He gathers the results and finds out that the majority of the class put their stops in the same place!
... at round numbers
... just below/above support or the low/high
... etc.
After years of testing students ... the professor forms a fund, maybe arbing the indexes to the underlying, and realizes he can extract a certain level of gain by exploiting this behavioral tendency and taking out the stops.
So, experiment. Maybe, try putting half your position on, then enter the other half at exactly where your stop would be. Tricky, because a stop turns to a market, so a limit enter on the opposite side may not behave exactly as predicted.
And, yes, if everybody starts thinking like this then the professor will figure it out again and exploit it.
"... see which way the herd is running and go the other way"
2 cents