Because he knew that all DADA teachers last only one year.
Snape could only go on working for both sides for so long; I think Dumbledore predicted that things were coming to a head and that Snape would need to make his allegiance to Voldemort clear by leaving Hogwarts. (As we saw in the first chapter, Snape working for Dumbledore was arousing suspicion.)
The more interesting question is, why has Snape wanted to teach DADA for years if he knew that all DADA teachers last only one year?
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Snape could only go on working for both sides for so long; I think Dumbledore predicted that things were coming to a head and that Snape would need to make his allegiance to Voldemort clear by leaving Hogwarts. (As we saw in the first chapter, Snape working for Dumbledore was arousing suspicion.)
The more interesting question is, why has Snape wanted to teach DADA for years if he knew that all DADA teachers last only one year?
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