Naturally, I don't agree with his sentiments, but I think the original poster intended to use the word "preponderance" rather than "propensity". I have never known the word propensity to be used as a collective noun. Thus I conclude that the original poster intended to ask -"Why is there such a preponderance of players on low/micro stakes, who have the propensity to play like total idiots?"
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They are a small minority, though, and I like playing low stakes sometimes