At one point Harry runs into Igor Karkaroff on the outskirts of the Forbidden Forest as Karkaroff heads in the direction of the dragons. Harry and Madame Maxime (and, consequently, Fleur Delacour) found out about the dragons via Hagrid. Yet Hagrid had no reason to tip Karkaroff off about the dragons.
The very existence of the dragons was supposed to be a closely guarded secret and the organisers of the Tournament would've taken pains to ensure that the arrival of the dragons at Hogwarts was as quiet and inconspicuous as possible. Nobody, as far as I can tell, intended that Karkaroff should know about the dragons. And yet he was somehow confident enough to take a night-time excursion in the Forbidden Forest in search of them (even if he didn't know he'd find dragons when he set out). It seems unlikely that it was a coincidence that he was in the right place at the right time.
How did he know to go looking for the dragons that night?