PwC Advisory Associate Interview

Interview Details – I apply from my university's job fair. And got an on campus interview with them. I supposed to meet with one manager, but he introduced me to another director. So I talked to two people on that day.
Two days later, they invited me to their office on Fri. I visited there and did three behavior interviews, plus some detailed questions based on my Resume. The whole process was smooth, and no difficult questions. But they reject me on Mon., I cannot think of any reason for that. Maybe they just want to find some cheap kids to work for them.