Transmissions tend to have a lot of things that move around. Gears might be ok for a long time by themselves, but when there's gear selection, you get extra wear and tear when things don't meet up quite right. And everything gets hot, so you need lubricant/cooling and that can get gunked up by the wear and tear...

And then sometimes things aren't built as designed. The ford HF35 hybrid transmission should be pretty reliable as designed, but many of them were built with bearings (or something) that went bad, leading to an early salvage date for many C-Maxes.