Shuddering is an involuntary convulsion that typically occurs when something is unconscionable or otherwise triggers a physionomic reaction that cannot be helped [meaning, it happens autonomously, of its own accord, based on the immediately preceding data]. You can't really shudder someone, it doesn't make any sense as a transitive verb, and yet the idiomatic usage in English has landed on "I shudder to X" where X is a thinking-pondering-considering verb such as "I shudder to think." or "I shudder at the prospect of his presidency" it's usually used to denote a non-positive feeling with the topic matter and when people actually shudder in real life, we do so with a sound exclamation under our breath such as "euuu" as your shoulders, arms, and potentially more gently vibrates or wiggles to indicate the effect being felt and received.
Human bodies shudder, and we typically denote body with "I" but it is a distinction between thinking self and bodily self, this shudder idea, because we typically don't use things such as "I was shuddering all night x" that's just weird and would make someone think you experienced a seizing episode. Not sure how this made it into English. In unrelated matters of similar linguistic domain, the Japanese have an expression "made my stomach stand" which is an expression to indicate "anger arose" ... not sure how that came to be either!
