Mysterious vibrating once a second

I saw some very strange behavior from my Verizon Galaxy S5 (running OptimalROM) this morning. I was watching a video at my computer with the phone (screen off) sitting on my thigh, when I start hearing something about once a second -- I pause the video, and figure out it is my phone -- I pick up my phone and discover the noise is actually the phone vibrating. It is vibrating for a full second, then nothing for a second, then it vibrates again for a full second, and this kept repeating. I turned the screen on, thinking there was an incoming call or an emergency alert, or something, but there's no phone call and no notifications of any sort. After the vibrating continued for about a minute, I finally decided to reboot the phone to see if that would stop it -- which it did, and it has yet to reoccur.

I've never seen this behavior from any Android phone, so I thought I would ask the community if they've seen it and know why it was doing it.