I apologize in advance for the very long and off topic post I am about to post.
SCR dimmers work by chopping the sine wave of AC. This is how they 'dim'. AC is a wave.
DC is a line.
Now when power is created, is is created by a magnet spinning in a coil at 60 times per second, thus 60hz AC. This is how and why the wave is created. The little sine wave above, the start as 0 degrees, the first peak point as 90 degrees, the next mid point as 180 degrees, and the bottom peak point is 270 degrees on our spinning magnet. This is what makes the wave. Motors take AC and use the wave to work. Picture a piston. It will be in the middle at 0 degrees, one extreme at 90, back to the middle at 180, and the opposite extreme is 270, then back to zero. It's this cycle that let's your piston move up and down evenly and predictably.
Now what does this have to do with SCR dimmers? SCR dimmers work be varying the voltage for your lamps to dim. At 50% on 120v AC, you will be using about 60v. 25% would be 30v, so on, so forth. It does this by chopping the sine wave. How it does this is by turning on and off the power every millisecond. So your wave will be flat, then peak, go down gradually to the next peak, then instantly flat again.
Now remember our piston and the circle, between 0 and 89.9999999999 degrees, no voltage, it sits still. Then all the sudden, peak voltage and it slams to the one extreme. Back to the middle at 180 and next extreme at 270. Then at 270.00000000000001, no voltage instantly and your piston is sitting in the middle again. This is very bad for the motor, because instead of working smoothly, it is getting extreme, sudden changes, and works much harder to do what it needs to do, using more amps, and heating up the piston and dimmer in the process, and heat it the enemy. It is what causes things to break.
A simple way to explain it, stand still, then start running full speed instantly, continue, then just stop just as instantly. Do this 60 times a second and see how you feel.
Now if the dimmer is is doing this, why don't your lights turn on and off while dimmed? Well, since this is doing it 60 times a second, it is not enough time for the lamp to warm up and cool down again, so you never even know this is going on. The buzzing noise that you hear when you dim lights or run your lights below 100% is proof of this. This is also why it's the loudest at 50%, because this is the worst the sine wave is chopped.
I have to go off on a gig now, will return to post some more on this later.