Karjakin and Fedoseev are the most obvious cases, but also Jakovenko and Morozevich have shown it from time to time.
Whenever something is happening, there is usually not one single clear cut explanation for it, instead there are a variety of possible explanations. Some are more likely, some are less likely. One of these possible explanations can be found >> here <<. How likely is it?
Note: I am not talking about possible doping, because using banned substances is only illegal if people don’t have a prescription for them.