Cavalry being knocked off of their horses isn't something new that was added by 1.8.0, though it made them more common.
Before 1.8.0 Beta, there were perks that gave heroes the ability to dismount enemy cavalry or knock down enemies if they hit them hard enough with the right kind of weapon.
1.8.0 Beta tied the ability to weapons instead of perks, while also making Athletics and Riding increase the ability to resist these effects. Basically if they get hit hard enough to bypass their resistance against the effects, they'll get dismounted or whatever. Cavalry getting dismounted is just easier to notice because troops actually use weapons that can do that, while weapons that can knock enemies down just aren't as common.