The default source/target levels are 1.5, which doesn’t support Java 1.7 syntax, obviously.
- http://maven.apache.org/plugins/maven-compiler-plugin/compile-mojo.html#source
- http://maven.apache.org/plugins/maven-compiler-plugin/compile-mojo.html#target
As to why it would “suddenly” change, something changed in your m2e or project configuration.