Snape and the DADA curse.

[Tonks]

No: HPFGUIDX 148859

Alright here is another idea that I haven't seen here yet. If someone 
came up with this and I missed it, sorry.

DD needs Slughorn to be at Hogwarts so that he can get the memory out 
of him. This is more important than Snape not getting the DADA 
position.  Since in order to get Slughorn DD has to offer him the 
Potions Masters Position, DD then naturally has to give Snape the DADA 
position.

Before he went to find Slughorn DD knew he would probably get him to 
say yes. DD told Snape that he could have the DADA position before he 
went to Slughorn.  In this way the DADA curse already had a chance to 
start and this is part of what caused Snape to get into the position of 
having to take the unbreakable vow.
 
Tonks_op