Check your static sag on both ends, especially the front. If your forks dived or you were bottoming out or had not yet rebounded from a bump or hole, the brakes can more easily lock up even with moderate pressure on the bike.

 

I've occasionally locked up my front wheel, believe it or not while demonstrating "emergency braking" as an instructor for an advanced motorcycle safety riding course. If your bike is perfectly vertical when you lock up the front tire and you let go quick enough, you can save it. It's scary as hell and it should teach you how not to do it again.
Riding on the street makes it somewhat difficult to "heat" up the tires and brake surfaces properly when you need to do a panic stop. The most efficient way to "briskly" stop a bike on the street is almost like applying your front brake through a "bell curve" with the vertical axis being brake force applied and the horizontal axis being time. You would squeeze your brake very rapidly just into the "friction point" of brake action. A split second later, you can squeeze very hard once the braking surfaces have heated up a bit and the tire has maybe made a rotation or two under load to get them warmed up more. Near the end of the stop just prior to actually coming to a stop (maybe the last few feet), you'll need to let up on the brake force a bit so you don't lock up the tire just before stopping. Maintaining the same brake force the last few feet is too much for the tire to handle as you no longer have the same forward motion and inertia and don't need quite as much braking force as just previously required. Obviously, on a loose or slippery surface, you'll need to moderate this action to a greater extent even though, for a panic stop, the same technique is still required.