(any idea why would it randomly switch to manual?)
Excel sets the session's calc mode based on the mode of the 1st workbook
that's opened. If the 1st book's saved mode happen to be manual, then it's
manual mode. And any subsequent book(s) that's opened within the same Excel
session (irrespective of their earlier saved calc modes) will "take-on" this
manual mode, and if saved thereafter would be saved on manual mode.