Three accounts active, two have funds.
Am migrating a contract using truffle.
Parity UI prompts me for the password for one of the accounts with funds. Why did it choose this one?
You can set the default for use by any application.
Go to APPLICATIONS and PERMISSIONS and toggle all accounts visible for external applications (blue bar = active and visible) and click the star to select a default (the direct answer to your question is: the yellow star)!