99% sure it was suffocation.
Duck tape can easily slide off from around the mouth unless wrapped plenty of times over.
He either overaped his mouth or overtaped his thorax.
I think he probably did both. He tied the guy’s hands behind his back pulling his shoulders and upper arms back. Tied in front his upper arms would give the chest a little buffer from the compression of the tape and clothesline.
Then he may have tied the chest real tight giving it little room to expand. He may have really gone overboard and wrapped tape/clothesline below the diaphragm preventing the abdomen from expanding and getting a breath that way.
Simply taping the mouth was bad. Most people couldn’t get enough breath through the nostrils when they’re excited. Combine that with restricting the chest and abdomen too much and that might be a perfect storm for suffocation. The article says the cops were there in ten minutes and he was already dead. Oh, and if he had asthma...