[PvK]

That is the usual behaviour for a fleet set to "Drop Troops (if carrying)". If the fleet has warships, they will fire at planets until it has no weapon platforms, and once they are gone, those warships will move away and the transports will move in and drop troops.

This will occur even if you let your ships break formation and use their own design strategies, as long as at least one troop-carrying ship has the Drop Troops primary movement order. The main requirement is that you have at least one ship with troops on it, and that ship needs to have the Drop Troops primary movement order.

The main exception I'm aware of is you also want to turn OFF the "fire at seekers" options, at least if you want to blockade a planet without attacking it, because otherwise your ships may move themselves into the range of planetary weapons just to take shots at seekers which would otherwise be out of range.

PvK