If the creatinine level is normal, what could be other reasons why urine bubbles when urinating?

That's normal: The wall of the bladder produces a little bit of mucin. Nobody knows why some folks make more than others, but this is the explanation for most bubbles. Don't worry about them.

Protein: It could be perfectly normal, but excess levels of protein being lost in the urine, which may be a kidney problem itself or signs of another condition could cause excess bubbles in the urine.