As commented upon by Jon Daniels (via ESPNDallas.com):
"Mike is in a key role for us and has had a prominent hand in our success the past few years. But this is a unique opportunity to discuss a coveted position with two storied franchises, and we believe in allowing our people to pursue opportunities that they're interested in. It reflects well both on Mike and our organization that he's under consideration."
As others have noted in the past, this is part of the unwelcome residue of winning -- other organizations end up wanting a piece of what has made you so great. Not that that has ended up proving true with Thad Levine and A.J. Preller yet, of course (though they've both been connected to every GM vacancy out there, and probably will end up being offered steps up at some point), but Maddux's stock has definitely risen in value since he arrived in Texas before the 2009 season, as he has closely overseen the transformation of the Rangers' pitching staff from one of the game's worst (performance-wise, at least) into one of the game's best.
[Apropos of nothing: Maddux was hired three years ago today, having been convinced to leave Milwaukee after Texas laid an offer on the table that promised to make Maddux one of the game's best-compensated pitching coaches.]
I'm not sure whether the idea would be to promote from within if Maddux were to go out and pursue greener pastures, but what I can tell you is that three years ago, the four main guys in the mix for the Rangers' pitching coach gig were Maddux, interim pitching coach (and now current bullpen coach) Andy Hawkins, Mariners special assistant to the executive vice president in pitching development George Wallace, and Rick Peterson, who already came and went from Milwaukee.
Given the relatively tight integration of the Rangers' organization as a whole (that is, scouting and player development not being treated like two wholly distinct entities), and given the number of pitchers on the staff that originated from the Rangers' farm system, I suppose we can't be surprised if the Rangers opt for somebody who is already familiar with and has already bought into the Rangers' current M.O. for the handling and development of pitchers.