I was in low sec next to an Amarr station. A player enrolled in the Amarr militia challenged me to a duel. I accepted. The player attacked me, and so did the station’s sentry guns. This was a surprise. I was not enrolled in faction warfare.
Can someone please explain in what cases sentry guns from a faction attack players that have not done a criminal action?
Sentry guns
Whenever you lose security status through suspect or criminal offenses in low or high security space, any sentry gun on grid will engage you. The sentry guns will continue engaging you for as long as you remain on grid.
No aggression is carried over after leaving the grid where you performed your initial act, so even though you’re still a suspect or a criminal there’s no further aggression from those or other sentry guns once you warp off-grid.
Limited Engagements are created on attacks towards other pilots that are under a suspect or criminal flag to allow them to defend themselves but can also be gained from accepting a Duel Request.
There’s a little-known mechanic about station sentry guns that, if I remember correctly, involves Faction standings. 
Congratulations upon stumbling upon very rare knowledge.
And that mechanic is?
That guy has bad rep. Pew pew on sight.