Craft periodically goes down due to plugin migrations
Okay so the site hasn't gone down in 2 days, so fingers crossed that we actually fixed this. 

Following Brad's initial comment, we noticed that the schemaVersion key was missing from that plugin's class file. We went ahead and added one:

/* this was there */
public function getVersion()
{
  return '1.0.0';
}

/* We went ahead and added this */
public function getSchemaVersion()
{
  return '1.0.0';
}


And now since the schema version === the current version, it no longer throws errors.