Rotation of the planet? Whatever it was that caused rockets to fall over.
Usually I saw that sort of thing when using big long rockets, when the physics kicks in sometimes they spring and wobble in weird ways, even if they're perfectly balanced. The... rocket holder thingy... helped with that quite a bit, but it introduces some of its own issues.
And of course it's random and unpredictable. That same rocket might load and stay perfectly vertical the second time.
Wheels and wheeled vehicles just sucked, the tiniest thing would make them explode for seemingly no reason.
I still play KSP from time to time, I have a hard time imagining how they make something... new out of it. Like, after you've gotten the idea of getting to orbit, controlling orbital distance, it's all the same thing. Whether I am carting parts to the mun, to minmus, wherever, it's the same basic loop. Launch out of the atmosphere toward orbit, establish orbit, play around in the maneuver planner until I get it to go where I want, follow the flight plan, tweak a bit to correct for the result, pass a bunch of time. Sure, landing on a planet with atmosphere and high gravity is different than landing on minmus, but it's the same basic idea.
I mean, that's not a knock on KSP, any game runs out of unique stuff to do. I just have a hard time imagining how you make that new again.
Being able to build and attach things separately would be fantastic, but mods do that now (poorly), and I won't pay full price just for some small features like that. On top of the fact that the new devs might just do a terrible job.