I think probably every racer who has dedicated substantial time to getting faster eventually finds themselves back against the wall of overtraining. I think for pro racers they would call it doing too many races. Regardless, it's the point beyond fatigue - often far beyond it.
HOW RACERS GET THERE: In the quest to get fitter, get skinnier, get FASTER, you must push your body beyond its limits, and push it past its limits regularly enough that it is forced to build systems that can regularly handle that workload. Once it gets to that point, your are a little bit faster. Your body builds those systems during periods of regular rest between the efforts.
The problem for some racers is when they become accustomed to that feeling of pushing the body beyond its limits, and associating that with going fast. And that if they're not doing that, they're not going faster. So they go, go, go! With eventual long term decline due to general fatigue that's not being addressed.
Another way racers may get there is too much enthusiasm. They're new to the sport and for the first several years really go full bore - train as much as possible, race as much as possible, research cycling as much as possible. And eventually it's too much.
SYMPTOMS: The power is just not there. When you get on your bike you are unmotivated. You are generally tired. You don't clean your dirty bike. You haven't had a rest or recovery day in two weeks. You ask yourself, "Why am I even riding bikes?"
CURES: Stay off the bike for a business week. Cross train - go on a run, do some weights.
Regardless, if you start to feel like you are overtraining, then you probably are. That means what you're doing isn't going to make you get any faster. Also, you run the risk of going so deep into overtraining that you'll get fed up and give up the sport. Why would you want to take that risk? Just take a day or two off and listen to what your body wants, not what your brain wants.