Couple of potential reasons i can think of, bad weather can sometimes damage ships, i found a merchant on my last patrol that was doing <1kt and listing badly thanks to a huge storm. But it seems unlikley that 2 ships would succumb to bad weather so thats probably not it.
Most likley they had come to the end of their waypoints and the "delete on reaching end of waypoints" option wasnt selected, so theyd just stop.
Theres more than a few random bugs that could account for this one, two DDs for free though, cant complain
