The Oracle Clusterware is designed to perform a node eviction by removing one or more nodes from the cluster if some critical problem is detected. A critical problem could be a node not responding via a network heartbeat, a node not responding via a disk heartbeat, a hung or severely degraded machine, or a hung ocssd.bin process. The purpose of this node eviction is to maintain the overall health of the cluster by removing bad members.
Starting in 11.2.0.2 RAC (or if you are on Exadata), a node eviction may not actually reboot the machine. This is called a rebootless restart. In this case we restart most of the clusterware stack to see if that fixes the unhealthy node