why did the jenova reunion happen to start just on the night that cloud and co. spent in the shinra building?

i mean for 5 years, nothing, and then it started all of a sudden.

the most obvious explanation is htat it has to do with jenova detecting cloud when he came near her body, but the question is, why wait for cloud?

the only reason i can think of is that sephiroth remembered that cloud was powerful enough to beat him, and that he figured cloud would be the best tool for him to get the black materia.