[Carorëiel]

I think a lot of things contribute to Pippin's breaking down at that point, but I think it's at least partly because he realizes the magnitude of his oath to Denthor. He offered his allegiance before he knew anything of Denethor's character or state of mind. He was then bound to serve Denethor even if he disagreed with him or found his actions appalling (i.e. sending Faramir back to Osgiliath). It's part of his maturation process. It may seem very romantic and noble to swear allegiance to someone in "payment" of a kind of life debt, but such an oath has very real, and possibly harsh, consequences. That kind of realization could be very overwhelming, especially given everything else that is going on for Pippin at that point.