We were shocked to hear that Billy Ray Cyrus filed for divorce last week from his wife Tish of 17 years, but the reason for the breakup of Miley's parents may be even more shocking: Tish was allegedly having an affair with rocker Bret Michaels!

It all started this past February, when Miley recorded the duet, "Nothing to Lose" with the 47-year-old.

Michaels apparently became "close with the entire family," a source tells Us, but not quite as close as he did with Tish!

In fact, Tish even pushed Miley to record "Every Rose Has Its Thorn" on Can't Be Tamed! In June, Miley told Ryan Seacrest, "Every time we go to New York we have to see Mamma Mia! for like the 12th time because my mom loves it, and that's kinda how "Every Rose" came about... she made me go to Rock of Ages, too! And my mom said 'I love that song, I forgot how much I loved it' and so then I did it."

"It was a professional relationship that turned into something more," says a source of the affair. And, "Billy Ray was completely unaware of what was going on."

Both Michaels rep as well as a spokesperson for Tish deny the dalliance, but there has to be SOME reason for the sudden split. And somehow, Tish and Bret just kind of make sense, don't they?

Billy Ray is asking for joint custody of the couple's minor children, Miley, who will be 18 later this month, Braison, 16, and Noah, 10.