..... Why would it fall while it was still in the oven?......
.......Was it because I opened the oven to take the other cake out? ....
Hope by now you have gotten the baking done and everything came out o.k.
There are sooooooo many reasons for a cake to fail....the recipe/mix; oven temp & time; etc, etc. Maybe the mix wasn't fresh; maybe somehow you didn't measure &/or
mix things exactly right; maybe your oven isn't working properly... so, so many differrent things to consider.
Because you opened the oven door? Most likely not, unless you left it open for more than 2-3 full minutes allowing the temp to fall dramaticly. You could have bumped the pan some how while taking the other one
out but even that shouldn't have made a big difference.