She's been interested in him a bit since he started doing little weird things she kept noticing. She probably was liking him and only grew to like him more around the time he laid those tissues out for her (just see the look on her face at the end of that chapter). She isn't suddenly falling for him this arc, that's dumb. She's already in deep at this point, she's just way, way more interested after this (and more importantly next) chapter.
I've noticed a lot of people try to put this in usual romance convention boxes, trying to figure out "when he started liking her" or "when she started liking him". The thing is, it's just gradual and a gradient of feelings, and there isn't a single point for either of them. Even Ichikawa's blatant moment where he admits it to himself isn't a turning point where he falls for her. He's liked her since before chapter 1, and was just misunderstanding his feelings. Yamada takes longer but it's still quite obvious that she's into him, not even counting the twitter comics which practically spell it out with the "subtext". At the very least by chapter 19, I think it's undeniable that she has a bit of a crush on him.