Originally Posted by SouthArk370Z

If it felt like a shudder or moan/growl, and the brakes are acting normally otherwise:
1) You picked up some water on your trip and causing grab/release cycles. Should disappear when things dry out.
2) The rotors have some glaze on them. Similar to #1 but it won't just go away. You will need to bed the brakes.
3) You have some oil or other contaminant (eg, Zauskycop's rust) on at least one of the pads. The pads will have to be pulled to inspect.
4) If #1-3 aren't the problem, start looking for mechanical problems (see B&W_Evader's comment above for a good start).
5) Some rotor/pad combos just do that (see Spooler's comment above), especially when cold.