Baked 2 cakes simultaneously. One sank in the middle on cooling
It sounds like you underbaked the cake to me, the structure hadn't crystallized and therefore couldn't support the weight of the case. Non-fan ovens often have warmer and cooler spots, the cake on the left was likely in a warmer spot than the one on the right, and cooked faster. Or the cake on the left was slightly smaller and cooked faster because of it, either way when you opened the oven door all the heat went out, lengthening the time it would take to finish baking the remaining cake and you just didn't give it enough time. 

Next time test the doneness of the cake using the appropriate method for that cake. I use a spring test and/or an instant read thermometer for most cakes. I don't usually use a toothpick test as I've found it to be inaccurate.