Most regulations are created via a rulemaking procedure described in §553 of the APA that is referred to as "informal rulemaking" or "notice-and comment rulemaking." In general, this process consists of 3 basic steps.
Notice - Agencies must publish a notice of their intent to create or amend a regulation. This notice is published in the Federal Register. The Federal Register is the daily publication for Rules, Proposed Rules, and Notices of the Federal Government.
The Notice of Proposed Rulemaking (NPRM) contains
Comment - Interested parties are invited to comment on the proposed rule. Agencies have discretion whether they want to hold hearings or accept oral testimony. More often comments are submitted to an agency through www.regulations.gov.
Final Rule - After receiving comments, the agency must publish a Notice of Final Rulemaking in the Federal Register. This notice must again state the reasons why the agency is creating or amending the rule and respond to comments received during the public comment period.
For additional details, see the Office of the Federal Register's Guide to the Rulemaking Process.
To understand why an agency created or amended a regulation, consult the preamble in the Notice of Final Rulemaking that was published in the Federal Register. The introductory part of the Notice containing the summary of why the agency is creating or amending the rule is referred to as the preamble, however, note that the word preamble does not actually appear in the Notice.
To locate the Notice of Final Rulemaking for a specific regulation, look at the text of the regulation in the Code of Federal Regulations (C.F.R.). At the end of the text of the rule, look at the source note or credit section. This will provide a chronological list of citations to the Federal Register (F.R.) for when the rule was first published and every time it was amended.
The Federal Register citations listed for this CFR section are 51 F.R. 22475, 54 FR 39633, 62 FR 51514, 80 FR 566144, 81 FR 49896, and 83 FR 46107. Every one of those notices would potentially discuss the change that you are interested in, although you may be able to narrow the list down by date.