Why does the poll ratings go down after a cabinet reshuffle?

I have had several games now where with only a few turns before the election I have had to reshuffle or sack ministers, and afterwards the poll has collapsed. Sometimes from a rating of 60% to zero! Why? IS this some sort of bug or is it meant to be realistic? Its just that such a drop when no other polices are touched seems ridiculous.

Any ideas?

Keith