Default classloader strategy is parent-first. Sonar libs are loaded before plugin libs. For this reason a plugin can not ignore Sonar libs or use other versions. It can generate conflicts in some cases, usually on XML libs.
A plugin should have the choice to use the child-first strategy, at its own risk. The property "Plugin-FirstClassloader" must be added with value "true" to the plugin manifest. It requires to improve the sonar-packaging-maven-plugin.