Revealing mistake: When Kelly enjoys her triumph at the middle of the stairs you can see a shadow on the wall right from her - obviously Al stands behind the wall waiting for his cue. (00:19:00)
Revealing mistake: Kelly is standing by the clock on the stairs. You can see Al and Peg's shadow on the wall waiting for Kelly to come up the steps, so they can come down. (00:02:59)
Answer: It's implied that Steve filed for divorce since in s04e16, "You Gotta Know When to Hold 'Em", Marcy says that Steve is suing for alimony. Alimony is something that has to be requested when filing for divorce. Although the show doesn't spend time on the actual legal proceedings of their divorce.
Bishop73