Your question is very general and it's impossible to give you a specific cause for the swollen glands.
Generally speaking, swollen glands in the neck can be a result of some type of infection in the fore-part of the body. If there are also swollen glands elsewhere on the body, that could indicate a problem within the lymphatic system of your dog, such as a lymph node cancer.
You need to consider a lot of other information about your dog's health and probably have some lab work done to determine the involvement of the swollen glands.
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