At night. Wet roads. Low speeds. Lots of changes in lane configurations. Open driveways to the right.
Probably not the best place to be trying AP. AP can handle some of those things, and can handle some of those things at the same time. Throwing all of those things in together at once is probably not a great idea.
Seems like the opening of the turn lane on your left, with the driveway that came up on the right, made AP think that the lane you were in was changing. The wet roads at night might have made the lane markings more difficult to see. You might have seen an indication on the display, but in that situation I wouldn't have been looking at the display to see it.