×You need to use movement order. If you right-click on the roaches, you can see the order they move each turn. Now suppose some roaches are lined up like this:

5-4-3-2-1

and they are trying to move east: the first roach will move east, then the second roach, and they will all move east. But if they are lined up like this:

1-2-3-4-5

then the first roach can't move east, because the second roach hasn't moved yet. So it will wait for a turn. And if that happens on the hot tiles....