Wow! Definitely brake with both front and rear at the same time. That's the way your bike is designed. If you brake with only front, you could endo. If you brake with only rear, you could skid. Apply both brakes evenly and adjust based on how your bike responds.

SnappyPix -- your wobble was likely caused not by bumping the bar/turning the wheel, but because you momentarily relieved weight from the front of the bike. When we descend, we should try to put a bit more weight on the front of the bike (one of the reasons you should always descend in your drops).

On of the biggest challenges with women's bike fit is that we modify man-sized bikes to fit our smaller upper bodies. We do this by adding a shorter stem. With older women and more recreational riders, we also put a stem with a large degree of rise, placing us in a more upright position. This combination (short, high stem), places too little weight on the front of the bike, which contributes to poor handling and speed wobble. If using a longer, lower stem isn't an option for you, try to consciously put more weight on the front of the bike when you descend, and I almost guarantee your speed wobble will disappear.