from 0.10 minute in the video i can hear someone following you and failing at trying to walk at same time as you in order to hide his noise. To me it looks like you were killed with ease and effortless, i mean you didnt even looked around you.

Next time you go back to loot yourself you should anticipate your killer to be nearby and waiting for you to go back to your corpse.

You seem to think it is hard to find good materials in dayz but in fact it is very easy if you put the time in the game. Its not because you dont know where or at what time to find grenades that other peoples cannot find multiple nades in a day playing dayz. Yesterday i found 3 nades while looking for an m4 wich i did found after the 7th or 8th crash site i did loot.

The unbearable ammount of loot you seen is a known bug related to item persistence, so far the fix is to turn persitence off but if you do then your tents will disapear next reset. It appears its exactly what happend to you, as the admin told you, they have put many hours in the game farming items, probably within their own persistent server.