The reason cats are afraid of umbrellas may be because they associate them with thunder and lightning. The shape and sound of umbrellas may frighten cats. Additionally, cats may view umbrellas as an offensive object and become fearful of them. In addition, some cats may be frightened by the shadow of an umbrella because they may associate it with the shadow of other animals.
In this case, you can try placing the umbrella next to the cat so that it gradually gets used to the umbrella and does not feel threatened. In short, the reason why cats are afraid of umbrellas is because of their instinctive reaction, which is related to individual experience and training methods. If your cat is afraid of umbrellas, try to gradually get him used to them without making him feel threatened.