Dynamic(ally typed) languages, by virtue of not requiring strict typing, often lead to more complicated function signatures.
Such functions are generally harder to reason about. Because they tend to require inspection of the function to see what is really going on.
Multithreaded code is incredibly hard to reason about. And reasoning about it becomes a lot easier if you have certain guarantees (e.g. this argument / return value always has this type, so I can always do this to it). Code written in dynamic languages will more often lack such guarantees, because of the complicated signatures. This makes it even harder to reason about Multithreaded code, increasing the risk posed by multithreaded code.