my cat has used my dogs bed as a bathroom spot three times!

my Siamese cat a little over a year old has recently decided to use our dogs bed as a bathroom spot the first time he pooped and peed in it and the other time only peeded we have imediatly washed the bed after. I just want to know what his problem is him and the dog are great together always playing. One thought I had was maybe it is a uti but it's not happining too frequently. What is the best solution and opinions? TIA!

I think that may be his way of stating his dominance over the dog, or "erasing" the dog's scent and leaving his own.  Are they having apower struggle?