A few times I restarted Yarn when there were running MapReduce jobs on it, but I found that the running MR jobs were not affected. i.e after restarting Yarn, the MR jobs could be resumed immediately, I was wondering that why didn't the MR jobs failed? btw, all of my MapReduce jobs were Pig script job.
The link provides the HA(High availability) architecture of YARN's Resource Manager.
In your case , I believe Automatic fail over is enabled ,so when the Resource Manager goes down , another RM is automatically elected to be the active.