Not sure it is a hypothesis unless the hypothesis is that the bug is either in subsection A or subsection B (and yes, there are the odd cases that both or neither is answer). Mostly the dissection is about reducing the space in which eventually either an exhaustive search (like stepping through) is conducted or a hypothesis is tested. Experience with similar systems and their architecture is extremely valuable for partitioning the system.

Whenever I form too early a hypothesis and test it I run danger of going down bunny trails. I may see something wrong but I can't be sure the wrong is not caused by other systems.