Hard to say why it's happening "all of a sudden" without more detail but if concerned I would call them to find out why.
I can only tell you that it's in CMS claims processing manual
Multiple procedures – When multiple procedures, other than evaluation and management services, are performed on the same day or at the same session by the same provider, the primary procedure or service may be reported as listed. The additional procedure(s) or service(s) may be identified by adding modifier ‘-51′ to the additional procedure or service code(s). MODIFIER 51 IS NOT REQUIRED FOR BILLING PURPOSES: The carrier will assign the multiple procedure modifier if appropriate based on the services billed. PAYMENT RULES: We approve 100% of the fee schedule amount for the highest valued procedure, 50% for the 2nd-5th procedures and “by report” for subsequent procedures. Payment determined on a “by report” basis for these codes should never be lower than 50 percent of the full payment amount.EXCEPTIONS: Multiple endoscopies: Special rules for multiple endoscopic procedures apply if the procedure is billed with another endoscopy in the same family. See CMS Internet Only Manual, Pub. 100-04, Chapter 12, Section 40.6 for endoscopy rules.