Typically, when a car is making ready to turn, as a cyclist you have a couple of courses of action, slow down and let the car turn and get clear from you path, go around the inside, or go around the outside, as you get closer to the car, you have to make a choice - you become more committed.
He had probably taken into account that you were going to turn and was ready for it, perhaps he was going to slow down enough for you to complete your turn and the road would have then been clear, but instead you did what he didn’t expect, you waited for him.That put a crease in his plans. He does not necessarily know you stopped because you saw him - some drivers do stop for no obvious reason then wait and turn anyway, if you had perhaps waved him past he would have been clear about your intentions as he then knows you have seen him.
Trouble is, he then doesn’t know what you intend, will you turn or won’t you? so he has to wait rather than risk going around you just in case you then turn in on him.
Another option is that you overtook him not long before you decided to turn, and you could have just rolled behind for a maybe 20 yards or so and made your turn behind him. This is a very common thing for car drivers to do, it is annoying for the cyclist who often has to slow down and lose momentum that can take some effort to regain, especially when the car driver gained virtually nothing - they feel like they have had someone just cut them up.