there was always an element of randomness in Japanese ASW. It could be very good on occasion. For example, USS Trout was sunk in feb. 44 by 3 veteran IJN DDs after it attacked a convoy. The big problem with the Japanese is they gave up the hunt too quickly.
On to BH's situation, an escort can detect you at 300 feet depending on various factors, i.e.:
http://www.ducimus.net/sh415/ai.htm
It may also be that the escort was dropping the DCs blind and just got a lucky hit. Even when an escort fails to detect you, it will still drop DCs on what it thinks is a likely spot.