Halliday & Matthiessen (2014: 314-5):
These enhancing circumstances range on a cline
(1) from circumstances that are like a feature of the process construed circumstantially, e.g. unsteadily (Manner: quality) in … He paced forward unsteadily …
(2) to circumstances that are like indirect participants, e.g. because of his asthma (Cause: reason) in Cam could not sleep because of his asthma.