While the timing sucks, I doubt this was a malicious or deceitful move by the program.
Programs come up for a site review on a routine scheduled basis. There is a huge lag time between the site visit and when the program gets notified of the outcome (i.e. months) - and they don't get any feedback of what the ACGME/RRC is considering in the meantime. So it probably was just unfortunate timing for the program.
As to the reason for it - Like
@Winged Scapula said, it could be something as minor as poor documentation/paperwork issues, or it could be something major.
Sometimes probation can actually be a positive, believe it or not. If there were real issues, nothing will force a program to change faster than an ACGME probation.