There are probably other reasons, but the main one is the grouping's ships are set too close to each other, and the "collision avoidance" is constantly engaged. The liner is probably trying to avoid the closest ship to it, so constantly helms away from it, while all the other escorts are trying to avoid the Liner... The game has a "minimum" spacing of 500 meters between ships, but that is center-point to center-point on the ships. If a ship is 500m long, it has already eaten half of that distance with its on hull. Add the 110m of an escort, and it is sensing an imminent collision constantly... pia to correct, but it is doable. SH3 has Hebe's mySH3-Tool that does flag all such groupings, but alas, that is not enabled for the SH4 game as of yet. We'll have to do some more, and stronger arm twisting...
