Crucifixion was indeed common. What I’m referring to as bizarre is the idea of the Messiah being subject to such a degrading, humiliating, torturous death, which ran totally counter to Jewish thought at the time (and indeed Jewish thought ever).
https://en.wikipedia.org/wiki/Sabbatai_Zevi
Nor would violating mainstream religious views be a terribly unique event when a new movement is picking up followers:
https://en.wikipedia.org/wiki/Jacob_Frank
It is also not as "bizarre" as you might think. Judaism two thousand years ago was highly fragmented, with many different movements all competing for followers and converts; there was no single body of "Jewish thought" at the time. There were different interpretations of the prophecies, and the earliest Christians followed one tradition of interpretations (this is clearly reflected in the gospels -- e.g. Matthew follows the Greco-Jewish tradition and the Septuagint).