I actually think they would fit in well. The lineup is: Ghost, Ministry, Annihilator, Overkill, Leprous (and some others). Seems like a good mixture of hard rock, prog, industrial, and some trash. Especially the combo of Sons of Apollo, Ghost, Leprous, and Annihilator seemed good to me. Sons of Apollo definitly shares elements, despite obvious differences in sound.
Obviously there is something amiss. Otherwise you don't cancel a good festival slot so short in advance, but I hope it is nothing serious or lasting. On the other hand, their other shows this week seem to be on track, so that is a good sign.