A couple of thoughts occur.

1) Vibration in conjunction with bad contacts. I had a car where the spring clamp mechanism that holds the base of the bulb (which is the earth) in place was worn and weak. The result was that when you drove the car, especially if you went over a bump, the bulb would bounce in the socket and momentarily flicker. After a remarkably short time, the low beam filament would burn out! It never affected the high beam. I put it down to current spikes combined with the vibration mechanically stressing the thinner filament.

2) One or more faulty diodes in the alternator. Alternators are three-phase AC generators, diode rectified to a not particularly clean DC. If one of the diodes goes out, you will get a fairly high voltage AC signal superimposed on the the nominally 12V DC line. A scope will show it instantly, a meter set to AC may well show it as an AC component to the DC voltage. If you can measure frequency with the meter, you'll see it change with engine RPM.

As Mark says, if the voltage regulator in the alternator was bad you may well also get more than 12-15v out. This would make the current going through a specific resistance, ie. a bulb, increase proportionately. The high beam filament may be man enough to take it, the low beam probably wouldn't. However, I suspect you'd also have all sorts of problems with other things working oddly/not working/smoking as well. Such as tail light bulbs, interior lights, etc. Note that a bad diode in the alternator would kill the regulator fairly fast as well, so we might both be right

At least in the UK, the regulator and diode pack are integrated into the alternator. It's easier and cheaper to replace the whole thing. If it is a bad alternator, eventually it will eat the battery as well.

pca