I don't remember anything specifically in game directly on point for the mages at Redcliffe, but I do recall that in the Western approach the Venatori mage makes mention of the ritual given to the Grey Warden mages to use had the side effect of making them mind controlled slaves.
I do remember Dorian talking about the Alexius distorting time to arrive in Redcliffe before you, and he notes that the alliance was "as if by magic, yes? Which is exactly right". It really isn't crystal clear that he's talking about the time travel here, it could be inferred that Fiona's decision to conscript herself and her fellow rebels to Alexius is what he is referring too.
That would make sense, they've already demonstrated that they have at least one form of mind control that works on mages and that they had no compunction against using it to recruit the grey wardens. It would also explain why Fiona would be so quick to sign off on such a deal rather than waiting and investigating other options first. I mean you later learn she's not only a mage, former grand enchanter but a former grey warden as well. It's not like she was some helpless babe in the woods so for me it seems reasonable that some form of mental manipulation must have been involved somewhere. I find it difficult to believe Alexius got her to sign herself and all of her fellow mages into indentured servitude in that short a period of time using nothing but negotiation.
Does it really matter that much? Eh.. no not really. A lot of folks on the mage side made bad choices. So did a lot of Templars, and a lot of Grey Wardens.. more than enough bad decisions to go around really. Of course if not, well the Inquisition wouldn't be nearly as necessary, now would it?
Ok, just my two cents worth. Back to the regularly scheduled cat fight. Enjoy