The preannouncement eliminates the possibility of a further delay, so that is new information. Also, as pointed out already, having the conference call is informative, as if all they had to say is that the study failed, then they would probably just have issued a PR. So probability of good results, or bad results and some major corporate restructuring, or good results plus partnership increases and probability of failed study alone decreases ==> change in current price. Nothing irrational in that.
EDIT: That said, I will still be worrying over the weekend that the conference call is to explain why the study "succeeded" even though not clinically meaningful or statistically significant in meeting endpoints, or why given the new information they should proceed even though they can't find a partner... you get the idea :-)
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