The phenomenon has been noticed for years. Much of the answer lies in the powder burn rate. With identical velocities at the muzzle in a given barrel length...the quicker the power, the more recoil it will generate with the same bullet mass...and the reason is largely theoretical, but seems to make sense.
Pistol powders reach peak pressure fast...often before the bullet has moved even a half-inch, and drops off quickly...while slower powders peak later and hold the peak pressure longer. With the faster powder, it seems feasible that at some point in its trip down the bore, it's actually moving faster than it is at the muzzle. Conversely, the slower powder doesn't do this...or at least the velocity difference between part-way and at exit isn't as great.
Since more force is required to accelerate a given mass to a given velocity in a given distance...if the bullet driven by the faster powder reaches a higher mid-way velocity than with the slower powder...and because force forward equals force backward...the higher the recoil impulse will be with the fast powder. This, even if the slow powder load produces a higher muzzle velocity.
Other factors are involved, of course. Bullet jacket and core material, and how much frictional resistance is offered to its passage affects pressures and force generated can have an effect. Bullet surface area in contact with the bore is another factor to consider. The more resistance the bullet encounters, the quicker the pressure rises and peaks.
It's also entirely possible for a given cartridge to generate more felt recoil with a lower muzzle velocity than another cartridge loaded with the same bullet, but with a slower powder.
The science of ballistics is long, wide, and deep.
