Its not manga magic timing coincidence, nor intentional/subconscious stalking done by one or the other.
Think about it, when in this manga has he talked to someone about anything other than her?
No crazy coincidence here, its just that he was always pronouncing his love to her to anyone who would listen, so anytime she was within earshot she would hear it.
Her mind then slowly transitioned from
Sorry, not interested -> just confess and get this over with -> this is embarrassing -> how long are you going to take -> just confess already -> why won't you confess -> I want you to confess -> do you really love me -> am I not good enough for you -> this is no good, your mine, why wont you become mine -> ....