As part of 7163848 the gc cause was added to all PrintGC logging for G1. It was suggested to do the same for the other collectors.

We do risk invalidating parsers with a change like this, but for the Full GC logging there was already "(System)" added to the logging when we did a System.gc(). So it seems lower risk to only add the gc cause to Full GC than to all GCs.
Suggested release note:

The log output when using -XX:+PrintGC will now list the GC cause in brackets if the flag -XX:+PrintGCCause is set. This flag is off by default in JDK7 and on by default in JDK8. Full GCs triggered by a call to System.gc() used to log "(System.gc)" or "(System)" when -XX:+PrintGC was on. Now this will only be included if -XX:+PrintGCCause is also on.