Full Idea
A 'deviant causal chain' is when an agent has a reason for performing an action, and for the reason to cause the performance, without that being the reason for which the agent performed it.
Gist of Idea
Deviant causal chain: a reason causes an action, but isn't the reason for which it was performed
Source
report of Donald Davidson (Freedom to Act [1973]) by Ram Neta - The Basing Relation II
Book Reference
'Routledge Companion to Epistemology', ed/tr. Bernecker,S/Pritchard,D [Routledge 2014], p.112
A Reaction
Davidson's thesis is that 'reasons are causes'. This was a problem he faced. I think this discussion is now obscured by the complex and multi-layered account of action which is emerging from neuroscience.