Ctrl-a does not behave!

This might be a bit off topic. But it happened while tweaking the model 100.

The thing is that all of a sudden, when I type ‘ctrl-a’ go to the beginning of a line or invoke the prefix in remote tmux-session, it instead prints out ‘^A’ (a caret-character followed by the letter A).

It seems this only happens in zsh. In bash and sh it acts as it supposed to.

Any hints are welcome because I am clueless and increasingly frustrated.