I think I'll go for the 'emotion-related' explanation for now.Now keep in mind I'm not 100% convinced he was able to use the "power" before either, but just to play devil's advocate for the validity of that possibility, the only times I can think of before this that we have to go by (i.e. that other sentient titans were around),
1) Eren himself was mindless (Trost)
2) We didn't get the other titan's POV (forest/Stohess with Annie)
3) Eren was able to scream (twice), but he was *interrupted* by blows so severe they put holes in his head (by Reiner in ch 42 or 43)
Say Reiner, Bert, and Annie suspected there was something special about him from the get-go, but again, he was mindless at the time and they couldn't be sure. This was their moment of confirmation.
If shifters require some kind of 'will' to activate their titan form, then perhaps Eren x Mikasa conversation triggers some kind of emotion in Eren ---> activating his 'special powers.' Good thing to consider is, Eren's desire to kill/destroy ---> vs. titan mob, vs. Annie, vs. Reiner (no ability activated?) Eren's desire to protect ---> vs. smiling Titan (protect Mikasa <--- hinted on the first page of Ch50), vs. Reiner (protect Scouting Legion, telling them to stay away.)
However, I must counter myself with the idea that "desire to protect" is not something Reiner would consider dangerous, but it's the closest thing I can think of right now.
I'm sure if he did have some form of ability activated back in the forest/Stohess, we would have seen a panel-ful of biribiri with Annie, or at least a hella lot "!!!!!" with her surprised titan face. Same with vs. RB. (Unlike DBZ, screaming does not grant you SSJ3 haha.)
I think the appearance of Eren is an important factor RBA consider. Or at least for now, that's the only thing we have been shown. There's also the physical features of Eren that they could have recognised. Also, from the outset I find it contradictory their foremost mission is to wipe out humanity in the Walls. I can understand by wiping out humanity, a formerly dangerous but dead Eren would not be able to use it. But I don't understand why they didn't kill Eren there and then when they captured him, because maybe their Plan B should override Plan A in case they think they have located the "coordinate." Or maybe they think Eren would join their side. It ultimately means Eren alive is of better use to them. The term "coordinate" makes me think as if there is something inside Eren that can be extracted, but I don't see how Christa would know about extraction of that something as Reiner and Bert guess. Or maybe what Christa knows is the form/shape the "coordinate" comes in.
Anyways, a whole load of guessed from me, there're just too many possibilities as far as I can see. I'm sure Armin or Hanji will give us a reliable guess that we can deduce better from pretty soon lol.
---------- Post added at 10:08 PM ---------- Previous post was at 10:01 PM ----------
Oh and also, I'm just guessing here: the range of his 'special power' is not known, but it doesn't seem the titan mob run towards him in particular. There are many other ones in the vicinity doing their own stuff. We also don't know if he could specifically affect one particular titan, or if he couldn't control the magnitude of his 'power.' It might be that RBA are out of range from his 'power.' We know the 7 titan in the basement of the gas-refill station do not notice Eren at all.

