I’m an ameteur home baker and I’ve only had 2 cake mishaps- both AFTER the cake left my possession. This past weekend my customer picked up this cake. The first photo is the finished product with remaining photos taken the next day at her party. I asked her if the icing slid off the side but the more I look at the photos I don’t know if I believe that’s what happened...since there is icing on the side of the box, the base of the cake is buckled...but this was the first time I used acrylic discs (which I think are a bit too large) so I may have had too much ABC on the cake-

what are your thoughts? 

This is just my guess I've never made a cake like that but I use buttercream all the time. It looks to me like the case got jostled in the car on the way home. I say this because it doesn't appear to me you have THAT much buttercream on the side and I'm assuming the cake was properly chilled. looking all the way to the other side of the cake it looks like it cracked/collapsed as in the car hit some bumps (the bottom next to the board past the other eyelash). I could say it melted if it was really hot but I don't think the picture shows that. Sorry that happened but I don't think it was your fault.

We've seen many similur posts.   As I recall most of us think it is the weight of all those roesettes that caused them to fall off.  Add the weight in w/hot/warm weather and/or fast driving and you have problems.

BTW: looking at your pictures I suggest in the future you cover all your boards.  It looks very unfinished and most of the time grease from the icing soaks in and stains them.