I'll be the first to say I ain't a fan of EC. Not that I'm an anti-fan, I just don't bother watching it.
It's hard to put one's finger on what exactly would make a show like Extra Credits condecending, but I can see why some people would feel that way. The general notion I've gathered from the few episodes I've watched seems to be that the guy/s behind the show knows a lot, or at least wants to seem that way. It's not up to me to say which it is, frankly I don't care. I think there's a general sentiment inherent to the style of delivery that says: "You ought to know all this, and if you don't we'll come out and tell you since you didn't bother to research this yourself". Is it what they're going for? Probably not. Much of what they talk about seems to be, to me at least, more common sense than "inside knowledge" from the games industry.
I think the EC guys really feel that games are an artform, and want it to evolve as such. While I don't have a personal bias for or against this argument, I think there's a general tendency in people who talk about art to get preachy. Probably in the same way that anybody who is passionate about something will become preachy on the subject. Doesn't make it a bad show. Personally, I gave up watching it 'cause the whiney voice filter gives me a headache and seems to be more of an annoyance than anything.