Shuttle and Reaver get focused down when the main attack squad is unsure of their attack plan.

[williums]

While they armies leave at the same time, the main squad sometimes realizes that they fail the combat simulation and therefore retreat to regroup with more units. The shuttle doesn't do this, and so the drop squad is focused down right away since our main army is dancing around and not drawing fire.

[jackykc]

I thought we want the shuttle to keep going regardless? We can make the shuttle attack from the other direction if it sees too much enemies that can damage the shuttle

[williums]

This usually happens for the initial drop. The first few zealots get to the base and see a million zerglings and back of until the dragoons show up to help out, meanwhile the drop squads proceeds to get smothered after firing one scarab. It's a terrible waste of resources.