How long is listed on the ACGME website, the link is in the above posts. Usually probation is for 6-12 months, either you fix the problem and get off or you get closed. Most programs fix the problems, at least on paper...
Why is not listed, and you'll need to speak to the PD. If you're worried you can contact them by email or phone first to decide if you'll interview there, but the PD is clearly going to put the best spin on it possible. Honestly, some of the ACGME decisions seem completely arbitrary. Sounds like clinical evaluations in 3rd year all over again....