[Sombre]

Yeah they're definitely trying to 'stay behind troops' and the reason is almost certainly that they aren't in range. When a unit can't cast a spell as it's been told to (or won't due to whatever crazy AI reasoning) it will check if it can cast any other spells instead and if it can't do that it will return to another default behaviour, which is stay behind troops. I'm guessing these two commanders can't cast any other spells (probably just indy commanders or something).

If you tell them to hold or cast that'll do the trick. Set their last order to cast spells though, so they hang around and keep going.