Why do we say "The fat is in the fire"?

Meaning:

Something that's likely to lead to trouble has started and looks like it can't be stopped.

Background:

This expression has been in use since at least the 16th century and was first recorded in John Heywood's still famous book of proverbs published in 1546.

Nowe if we awarde me this wydowe to wedde,
And that I dryue of tyme, tyll tyme she be dedde:
Than farewell ryches, the fat is in the fyre.
And neuer shall I to lyke riches aspire.

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