The "UNABLE TO LOCK ROW" error typically occurs when multiple processes attempt to update child records that share the same parent record at the same time. In this case, when importing Contacts associated with Accounts, the Data Loader is likely attempting to update several Contacts for the same Account simultaneously, causing row-level locking issues.
Salesforce's documentation explains that row locks can occur when related records are updated concurrently.
To avoid this, it is recommended to batch imports so that child records with the same parent are processed sequentially rather than simultaneously.