The package has a trigger that Lintian does not recognise in its control file.
The package may be uninstallable if dpkg does not support the trigger.
If this is a new type of trigger, then Lintian may simply be out of date. Please verify if the latest version of Lintian knows of the trigger, or file a bug against Lintian.
Refer to the deb-triggers(5) manual page for details.