There can be a lot of variables to this question.
The grooming process is not as relaxing for your pet as you might think. Some dogs love the attention, the bath, the brushing, while some dogs despise getting washed and combed out. Some dogs see grooming as a part of life and some dogs see grooming as a sort of punishment.
For grooming, dogs are working just like the groomer is.
Standing in the bath tub, standing for the drying process, and standing for the a big portion of the groom can be taxing on your pet especially if they are over weight. When the groomer has to do their nails, shave their paws, trim around their feet, your pet has to be able to balance a large part of that time on three legs. Now their other leg muscles are staying engaged in doing so, so that they do not fall over. Your pet is mentally focused on the groomer just as much as the groomer is focused on your pet. It can be beneficial to your pet that they get adequate breaks in between the grooming process to allow some downtime or time to mentally relax through the stimulation in the grooming salon.
Salons are noisy with the equipment that is needed to be able to execute the job at hand. High velocity dryers, vacuum cleaners, clippers, dogs barking, telephone ringing, ect. can be overly stimulating to your pet. Especially if they are used to living alone with just you and maybe only a TV playing at home.
Dogs also can have separation anxiety when you are leaving them. Sometimes the only time the pet is away from you is when they do go to the groomers and that might only be once every 6-12 weeks for a handful of hours.
The best way to handle a nervous pet is to be confident yourself in leaving your pet in your groomers care. Your dog can feel your energy levels and if you are nervous and upset, your pet will pick that emotion up and in turn will be nervous and scared. The pet looks to their owners for guidance and confidence.
These are just a couple of examples on why your pet may shake at the groomers.
Attached is a link to a blog with some more information on the subject: https://www.powderhoundsgroomingsalon.com/post/why-is-my-dog-scared-to-go-to-the-groomer-s-dog-groomer-q-a-s
Always feel free to ask as many questions as needed to ensure you are as educated and informed on what happens at any grooming salon.