Newly adopted or puppy dogs may not be house-trained. They may not realize that the bed is not a toilet.
Dogs, especially puppies, may urinate when aroused or submissive. Excitement or submissive body language like squatting, grinning, or tail wagging often accompany this activity.
Cognitive impairment or other age-related disorders may impair bladder control in older dogs. They may urinate in their owner's bed accidently.
Separation anxiety, boredom, and lack of exercise and mental stimulation might cause dogs to urinate indoors.
Dogs have great senses of smell and may urinate on their owner's bed to mix their scent with theirs for comfort or security.