Via TRAC:

"Given the high volume of traffic of Canadians and Mexicans into the U.S. for many reasons (e.g., work, visiting friends and relatives, shopping, relaxation), it is not surprising that citizens of these two countries rank towards the top among these inadmissible stops. Inadmissibles from Mexico top the list with 736,501 (24%) and inadmissibles from Canada rank third with 284,308 (9%). But ranking above Canada with 386,006 (12%) are people from the Philippines."

"The specific reason individuals are stopped as inadmissible are included from CBP’s complete list of ~70 reasons used. The corresponding equally detailed reasons for the OFO officer’s decision on how each individual should be then handled (“disposed of”) that provide the justification for turning them away, or letting them into the country, is available. Reasons include, for example, the specific reason for awarding parole, such as the Cuban Adjustment Act, Central American Minor, Operation Allies Refuge Parole, Haitian Family Reunification, Filipino World War II Veterans Parole, and the Ukrainian Humanitarian Parole/Uniting for Ukraine."

https://trac.syr.edu/immigration/reports/692/