If you don't use Class.forName(...) to load the driver manually, then I think you faced an infamous problem with maven-assembly-plugin - it overwrites files with the same name when they come from different jars.
In your case JDBC driver discovery mechanism relies on a file named /META-INF/services/java.sql.Driver, and you have at least two jars containing such a file in your dependencies (Oracle and Postgres drivers), therefore one of them is lost after running maven-assembly-plugin.
You can use maven-shade-plugin instead of maven-assembly-plugin to merge these files correcly, as described here.
Alternatively, you can use Class.forName(...) to sidestep the failing autodiscovery mechanism.
