As far as I know, neither Bruce Waltke nor RTS has made a public, official statement on the reasons for his resignation. Given the speculations on the web, I thought that I would post what one blog commenter had to say:
I’d like to offer a couple of clarifications:
1. Some are suggesting that RTS asked Waltke to leave (you didn’t) over the video. It seems to me like Dr. Waltke is simply retiring more completely. In a recent email exchange with Dr. Waltke, he seemed very tired and frustrated by some of the ongoing things in evangelicalism (both those to the left and right of him). I don’t expect him to retire from academia, but I don’t think RTS forced him out since his video was clearly in line with the thinking in his 2007 OT Theology (which is excellent for anyone interested).