Why was Snape teaching DADA?

[oiboyz]

No: HPFGUIDX 135906

Another possible reason: DD never let Snape teach DADA before because 
of LV's curse on the job, which he admits in HBP is real.  If he gives 
Snape the DADA job, Snape might end up attacked by centaurs or losing 
his memory or locked in a trunk all year or whatever horrible thing is 
destined for the DADA teacher-- and DD doesn't want to risk that; he 
wants to keep Snape around.

   But in HBP Snape has taken a vow that he'll either have to break or 
fulfill by the end of the year, meaning he'll either be dead or he'll 
have to flee.  Since he'll be gone anyway, might as well give him the 
DADA post.