Hrm... So far best I can come up with are two possibilities, one more charitable and one less charitable toward Snape:

First: From his conversation with Harry, he perhaps concludes that making one's feelings (or lack theirof) to someone that may have feelings for you absolutely clear rather than leaving it ambiguous may be a good idea. (ie, in his own way, he wanted to protect his student from ending up in a situation similar to himself)

Second: He was sufficiently POed at Harry that he's no longer keeping his agreement to not legilimens the students.

I'm not really assigning all that high credence to either of those hypotheses, but they're more or less all I could come up with so far. So... give us a hint? :)