Out of app memory - what happened?

This is the first time I've ever seen this happen! A Yosemite bug?

I'm using a late 2013 MacBook Pro 13" retina with 16 GB of RAM and 512 GB SSD.

This morning while composing a mail in Mail.app, I suddenly got a dialog popup saying "Your system has run out of application memory" and telling me to quit applications I am not using and a bunch of apps went unresponsive.

I tried force-quitting all the paused apps, and then other apps, but it didn't help. I needed to restart the computer. This is what the Activity Monitor was showing in the memory tab at the time.

The obviously big one is Mail - requiring 63.96 GB of RAM!

Is this a Mail bug in Yosemite perhaps?

I understand the Google Chrome Helper messages are well known and are considered "cosmetic" right now, so I don't think that's the cause.

After restarting, with all the same apps open, with all the same tabs and files open, things now look normal again.

But I wonder what happened. Anybody else have this happen?

Thanks,

doug