Research tells me this is the correct reason, though I don't actually know precisely how it makes this happen... it just does.
Somebody please feel free to add technical detail where this is lacking.
A mains supply transformer from 120/240v AC to some small voltage DC needs smoothing. This is done using a rectifier.
Consider that the AC is like a sine wave with peaks & troughs, the current literally flows in one direction then the other, 50/60 times a second.
The rectifier's job is to make the current flow in one direction only - hence DC.
There are two types of rectifier, half & full
A half rectifier simply 'removes' the lower half of the sine wave, keeping only the top, flowing in the correct direction
A full rectifier 'mirrors' the negative portion into the positive & keeps the flow more smooth.
A half rectifier is what makes your phone screen jumpy - makes it uncertain where you touched or with how many fingers.
The only fix is to use a full rectifier instead.
Image credits & a much better explanation on Wikipedia: Rectifier