The way you have your SQL Server configured:

Says that 4 GB will be left for non-buffer pool memory. This includes some memory (for things mentioned earlier) used by SQL Server, but also for the OS. You need to find out what else is using memory on your server. You could back down the max memory setting from 8192 to something lower, but chances are whatever this thing is, it will gobble up whatever else you leave open for it to grab anyway so best to figure out what it is. What you can do is, setup a PowerShell script to run every minute that stores the list of all OS processes and how much memory they are using. Then, when you encounter an issue like this you can refer to that list and see the culprit. Maybe your monitoring software, the one sending you the alerts, is already doing this or can be setup to do it without writing any code yourself.