Why do we say "When the cat's away, the mice will play"?

Meaning:

When the person is in charge is away, the subordinates will take advantage of the situation

Background:

The essence of this expression can be found in a number of languages. In English, an early use in something resembling its modern form can be found in Shakespeare's Henry the Fifth (1599), Act I, Scene II:

Westmoreland, speaking with King Henry V, Gloucester, Bedford, Exeter and Warwick
But there's a saying very old and true,
'If that you will France win,
Then with Scotland first begin:'
For once the eagle England being in prey,
To her unguarded nest the weasel Scot
Comes sneaking and so sucks her princely eggs,
Playing the mouse in absence of the cat,
To tear and havoc more than she can eat.

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