I think that the announcers get this "information" in the form of complaints from players they talk with before the games. When someone is going bad, it's understandable that he will blame an array of factors and among the more obviously available choices are the umps. The pitcher who is walking a lot of guys is getting squeezed by the ump, the batter who is striking out is the victim of an ump's "expanded zone."
The announcers pass along these perceptions as if they were facts. And after passing x number of such complaints along, they decide to codify matters by proclaiming that umps are inclined as a group to stick it to struggling players. It becomes conventional wisdom and people forget to really think about it.
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