When someone suddenly acts out of character, what could it be?

MercuryKitten
What are some reasons, in your opinion, why someone who was very shy and socially awkward, and yet sweet, kind and considerate could suddenly act completely trashy?

The only thing I can think of is drugs.

Does anyone else have some insight? I need help on this one.
When someone suddenly acts out of character, what could it be?
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