This post is in the “that is interesting category.” I am reading through John’s Gospel. One theme that I am noticing for the first time is how many times John tells us that many people believed in Jesus.
John 2:23 Now when he was in Jerusalem at the Passover Feast, many believed in his name when they saw the signs that he was doing.
John 4:39 Many Samaritans from that town believed in him because of the woman’s testimony, “He told me all that I ever did.”
John 4:41 And many more believed because of his word.
At this point, my mind goes to John 6:66 where we are told many of disciples left him after his speech on eating his flesh and drinking his blood. I assume that after John 6 his following was reduced as many left him because of his hard teachings. Yet the converts keep coming. The refrain “many believed” is found over and over again in the chapters leading up to Christ’s final days.
John 7:31 Yet many of the people believed in him. They said, “When the Christ appears, will he do more signs than this man has done?”
John 8:30 As he was saying these things, many believed in him.
John 10:42 And many believed in him there.
John 11:45 Many of the Jews therefore, who had come with Mary and had seen what he did, believed in him
John 12:42 Nevertheless, many even of the authorities believed in him, but for fear of the Pharisees they did not confess it, so that they would not be put out of the synagogue.
Five times after John 6 we are told that “many” believed in Christ. I am not sure what to make of this. My assumption has been that Christ was rejected by all with even his disciples leaving him on that last night. While he was certainly rejected, John emphasizes that there were still many who trusted in Him as he preached and worked miracles. It makes me wonder if those who called for his blood on Good Friday were from Jerusalem while many around Israel still trusted in Christ.
I think you're definitely on to something, Peter.
Kevin DeYoung points out that the crowd shouting “Hosanna” was not the same crowd that shouted “Crucify Him.” 
This could also serve to explain Jesus' lament over the city of Jerusalem specifically. The city (and it's people) specifically had rejected him. He has similar condemnations for Chorazin and Bethsaida. He was there among them teaching and preaching and they still rejected, much like they did in Jerusalem. While Jesus did experience rejection, we can't assume it was too wide-spread, because those who arrested him had to plot and plan to do it in secret so the city wouldn't riot.
Chris, I remember that post by DeYoung. I think we get sloppy with our exegesis sometimes because we want to make Christ's rejection sound as bad as possible. Of course, the main rejection was by the Father. “My God, My God, Why have you forsaken me!”
Seth, you are right. Why the concern for rioting if no one was following him? Herod had a similar concern about killing John the Baptist (Matthew 14:5).