I've seen this happen when one of the convoy's ships is stopped dead in the water(or going slow). The rest of the convoy decides to wait for him, and the party is ready to begin
It looks quite funny when a convoy of over 20 ships tries to find their position around their damaged comrade.
Other times, when damaged ships aren't around, the escorts seem to imitate Bungo Pete and stop to listen. This used to happen quite often in 1.3 when several escorts were searching for you, but isn't imho so common in 1.4.