JJ saw the mine fly backwards, as he was looking up at a consistent rate to make sure he didn't accidentally reveal himself to the blast. He peeked around the corner, and saw where the mine hand landed. So, he barely revealed one of his shotguns poking around the corner, and he fired at the mine. This caused the mine to detonate with JJ safely away. JJ continued onward, following Cannon's footprints in said muddy trench, tracing them back to the bunker and its shut door. So, he gets some distance, and uses a densely packed bunch of ash to open the door, and then he takes cover before something can happen.