Was it a price drop of the same ticket or just that availability in a cheaper fare appeared then disappeared, which is most likely to me (ie were some €500 tickets always available for some different dates just not yours).

If the latter, there is no "when" really. As with all airlines, an algorithm (to grossly simplify) compares the level of filling and prices paid of a given flight to "expectations" every day, and on the basis of what it finds will decide to open or close seats in any given buckets. So it looked like at one point, the system decided to open seats in a very cheap buckets and then people bought more tickets and the prediction changed again.