Why did the stock of consumer loans held by commercial banks jump in March 2010? The graph below refers to "Consumer Loans: Credit Cards and Other Revolving Plans, All Commercial Banks", from here.



What happened in March 2010? The data come from the balance sheets of commercial banks in the US. Did an entity that was previously not a commercial bank somehow change status?
Because of a change in regulation about how different accounts are to be recorded. That change was stated in the Financial Accounting Statements No. 166.

If you look into Notes on Data relating the "Assets and Liabilities of Commercial Banks in the United States - H.8", you find the following statement from April 9, 2010:


  As of the week ending March 31, 2010, domestically chartered banks and foreign-related institutions had consolidated onto their balance sheets the following assets and liabilities of off-balance-sheet vehicles, owing to the adoption of FASB's Financial Accounting Statements No. 166 (FAS 166), "Accounting for Transfers of Financial Assets," and No. 167 (FAS 167), "Amendments to FASB Interpretation No. 46(R)." Domestically chartered commercial banks consolidated \$363.4 billion in assets and liabilities. The major asset items affected were: other securities, mortgage-backed securities, -\$6.4; other securities, non-MBS, -\$23.5; commercial and industrial loans, \$19.1; real estate loans, revolving home equity loans, \$5.5; real estate loans, closed-end residential loans, \$20.9; real estate loans, commercial real estate loans, \$1.2; consumer loans, credit cards and other revolving plans, \$321.9; consumer loans, other consumer loans, \$24.6; other loans and leases, all other loans and leases, \$46.1; allowance for loan and lease losses, \$36.4; cash assets, \$4.1; trading assets, other trading assets, -\$1.7; and other asset items, -\$11.9. The major liability items affected were: borrowings, borrowings from banks in the U.S., \$2.6; borrowings, borrowings from others, \$379.8; and other liability items, -\$1.6. The residual (assets less liabilities) decreased \$17.4. The major memoranda items affected were: securitized consumer loans, securitized credit cards and other revolving plans, -\$322.7; securitized consumer loans, other securitized consumer loans, -\$24.5; and securitized real estate loans, -\$23.9. Foreign-related institutions consolidated \$20.7 billion in assets and liabilities. The major asset items affected were: other securities, non-MBS, \$1.1; commercial and industrial loans, \$0.7; and other loans and leases, \$18.9. The major liability items affected were: deposits, other deposits, \$0.5; borrowings, borrowings from others, \$21.0; net due to related foreign offices, -\$12.0; and other liabilities, \$11.2.


I highlighted the one corresponding to the graph you show. The magnitude of the jump precisely matches the amount the statement above mentions.