I do not understand what using the term "rat" has to do with seniority. Could someone please explain this to me?

It doesn't really have anything to do with seniority. It just looks a bit funny coming from lesser experienced umpires who have not had to deal with a lifetime's share of rats yet. Don't take offense to it, but it's just that you ain't seen nothin' yet, to quote Bachman-Turner Overdrive.

Offerred in the spirit of learning ...

I seem to recall that not too long ago you had to ask what the term "rat" meant. Now, in this thread, you seem to be throwing it around with impugnity. You might not be using the term correctly.

I don't recall seeing any rat-like behavior in the video (not that it might not have been there; not that there might not have been some if I'd been on the field).

I few weeks ago I asked for a definition of jock sniffers, but I don't recall asking for a definition of a rat. From hearing the term used for almost 2 years now, I take it to mean a coach (or player for that matter) who acts like a jerk towards umpires. This leaves room for a broad range of behaviours. Please correct me if I am wrong.