why did everyone think he betrayed them?

Watching it (again) and they think he may be connected to a clocksmith who is capable of changing them. Not him himself.

Certainly easier for them to blame a person than to admit that either
1) the robots were capable of spontaneously evolving past their protocols, or
2) when the original unbound robot designed the encryption for the protocols, it entered a tiny random chance of the protocols being overridden, so that the robots could evolve eventually.

I'd put my bets on #2, since the first robot mind had already advanced in intelligence past humanity, and as developing the encryption was it's last task, it probably had some clue as to what was coming.

they didn't think a robot could do those things so they thought it was jack who did it

--------------------
Krillin: Man, that android is toast!
Piccolo: Yeah... More like, burnt toast.

It didn't make any sense to me. Why would they think he had the capability to alter the protocol?

[deleted]