i will be the dissenting opinion on dowels here .... if you used 5, AND they were pencil thick, then you were fine with the number that you used. I would have only used 4. I only use 5 if I have a 4 tier cake. Otherwise, it's 4 dowels in every tier, regardless of size. Did it that way for years. Only had one slider in 30 years and that was because I had to slam on my brakes. the cake autopsy showed I had too much rasp filling, which contributed to the slide.
I had the "honor" of cutting a cake that a baker had filled with dowels. Way too many.
The cake was like swiss cheese ... a crumbly mess, difficult to cut, and did I mention it was a freakin' mess???? Too many dowels will damage the structural integrity of the cake.
were you there when the cake was cut? Did one of the dowels slip out of place? by looking at it, that would be my guess. A top tier sliding can push/pull a dowel out of place, which removes the support under that spot in the cake, which can cause a collapse, like I'm seeing in the pic.
Agree with the center dowel for any cakes needing transported already assembled.