Changing default shell

Hello,

Whenever I try changing my default shell to zsh, it works fine:

(baobab)-[cepaitis@login2 ~]$ chsh

Changing shell for cepaitis.

New shell [/bin/bash]

/bin/zsh

Password:

Shell changed.

However, after a few weeks, it changes back by itself.

(baobab)-[cepaitis@login2 ~]$ echo $SHELL

/bin/bash

Do you know what might be going on?

Best regards,
Vilius