When using the leg press, I found that during a one leg press, the glutes are used more when opposite leg is down, and the quadriceps are used more when the opposite leg is up. Why is this?
This is interesting. Before we cover why this may be, it would be wise to ask yourself why you are performing a single leg press. Leg presses are a tool for isolation, which usually means "heavy/maximal" loads with hypertrophic goals, relatively. Because of this, the maintenance of NEUTRAL SPINE (NS) is VITAL to insure no compensation and/or repetitive stress leading to injury. Performing leg presses in a single leg fashion makes NS difficult to maintain due to the unilateral fashion of the exercise. Generally speaking, when ever an exercise is performed with only one extremity at a time, there will be an element of rotational force(s) introduced into the movement that must be stabilized (depending upon the goal of that exercise). Heavy/maximal loads in a leg press may not be the best place to introduce these forces to the body/spine.
Now, back to your original question. I have found that what muscle one "feels working" when exercising can be very relative or subjective to "intent." If the trainer cues the client to concentrate more on the glutes (hip extension), rather than the quads (knee extension) this can greatly alter what they "feel." Paralleled with that, if you as the trainer cue the client to focus more on pushing through the heels rather than the forefoot, you can slightly change the mechanics of the exercise so that the glutes with be doing more work.
Another item to consider is foot placement. Look at the diagrams below. As DIAGRAM 1 shows, if foot placement is high, the load line is closer to the knee and further from the hip placing more stress on the hip extensors (glutes). Conversely, in DIAGRAM 2, if the foot placement is low, the load line is closer to the hips, placing more stress on the knee extensors (quads). (NOTE: The RED arrows depict the load line.)
NOTE: These diagrams were referenced from "The Physiology of Movement" by Slavin