Those bubbles are water getting in between the fiberglass and the gelcoat of the steps. It is from the way thy wwere made, not from anything you did. It is called delamination. There isn't anything that can be done except to refinish the steps - but I wouldn't worry about them yet. Keep your eye on them, and see if they get worse. I have had them in my fiberglass pool for two years, and eventually will have the pool resurfaced.