The fan base actually booed Frost back in 1996, which is why people assume the message boards would have been ridiculously vicious towards him.
There is a segment of the fan base which to this day has never forgiven Frost for leaving Nebraska in the first place, and there was still plenty of bitterness about that in 1996 when he was back and won the starting job. That anger was simply additional fuel to the fire that began with the Arizona State game that year, when we were shut out 19-0. We were shut out by a team that we had scored 56 points on in one half back in 1995, and although it had more to do with the center (who kept snapping it over Frost's head into our own endzone) and our offensive line than it did Frost, the fans of course came down on the QB. Not to completely excuse Frost, who had a bad option pitch that ended our only sustained drive of the night.
Even though Frost only lost one more game in his college career after that (the first Big 12 championship against Texas), the fans were unreasonably hard on him, actually booing him at home. The only other time I can remember Nebraska fans booing their own would have been 2007. Anyways, I don't think Frost was able to win over the majority of the fan base until he helped defeat Washington in 1997, and of course by the end of that championship year he was everybody's golden boy.
edit: to answer your questions specifically - it wasn't about him getting the job, although people were constantly clamoring for the backup (Frankie London) as fans are prone to do when they're not happy with the starter.
I don't think it was anger that he came back, though maybe people thought he was just jumping on the bandwagon after 1994 since we started winning NC's. But it was mostly due to the out-of-control expectations Nebraska fans developed after 26 straight wins engineered mostly by Frazier and Berringer (2 of our best QBs ever) and also due to the fact that people were offended Frost had left the state to begin with.
