He suffered from diabetes which, combined with the rigors of the life in the jungle, had caused painful injuries on his feet. That was the reason why he had to order a new pair of special boots. And that was when Operation Sodoma started and his death begun.
The guerrilla command sent a message asking for these special boots, which was caught by Colombian intelligence. The Colombians were able to intercept the boots and rig one of them with GPS circuitry. When Mono got them, his fate was sealed.