Dan Rather, infotainer. The man loves to cry on camera. He did it on Letterman shortly after 9/11, he did it numerous times on-air when he was the CBS anchorman...it's part of his schtick, like his faux-homey colloquialisms or the v-neck sweater he used to wear under his suit. The crying on cue thing is in fact the lamest aspect of Rather's schtick, and that says a lot.
I believe that Rather practices crying. I'm not kidding. I think the man works on it.