Who knows but check the site below for different reasons cakes fall. You can't wing it as much with cakes since it's pretty much science to be successful.

Here is a link that might be useful: Why cakes fall

I would imagine having too many pans could easily have been the only reason. I think you would have needed to bake a little longer for them to be completely done

I made 16 in cakes, 12 in & 8 in cakes for a wedding All Duncan Hines & all looked perfect. I went to flip them over unto cake boards & 1 after another they turned into pile of crumbs.(1970's I think) I called DH & they sent 6 pages of stuff on how not to have a flop with their cakes. I told them to go back to their old dependable receipe. They refused even tho cake bakers were writing "Maid of Scandinavia's national cake decorating magazine complaining. Duncan Hines flatly refused to change it back. They lost a lot of valuable customers. I've never bought another 1. Later Coke tried changing their formula & quickly saw error of their way & changed it back. I've had oven full of cakes, most bakers do can't get cakes done & have them fresh if only doing couple at a time. 1 reason your cake might have fallen, well 2- 1. did you try to move them around in oven or even pull the rack out(don't have any racks in oven but 1 cakes are on so you can test with toothpick by reaching in with hand) when part way done- that will cause a flop with any cake from scratch or otherwise. 2. did your toothpick come out clean as a whistle, with the water & that many eggs if it isn't done it will sink!! That would be my assessment!!Jan