Bizzare Knockdown Behavior

So I invaded this guy from Austria in the forest earlier. I rushed up and parried and riposted his SKSpear, then hit him again as he got up. He teleported away and started chugging estus, so I ran him down and backstabbed. As he gets up he pulls out a catalyst and dark beads me. 

This is where things get weird. The Dark bead knocks me into the floor. Like the Zwei R2, but for pathetic damage (for 1/4 of my hp). 

Now, I've PVPed all this while inductively concluding that knockdowns granted invulnerability (i.e DSlayer arrow knockdowns give invincibility, BS knockdowns give invincibility, Zwei/demon greataxe knockdowns give invincibility) but he instantly followed up with another R1 dark bead and it took off most of the rest of my hp while I was on the ground. This second dark bead also resetted me into a standing 'stun' animation. 

Following this I rolled out behind him, and he dark beaded the empty space in front of him where I'd been 2 seconds prior, instantly lagshot killing me as I recovered from the roll behind him. I had 9 poise.

I'm confused about a few things.

1) Do knockdown animations (as in, where your character ends up falling to the floor) always offer invincibility frames?
2) If not, what weapon/spell knockdowns cause knockdown with no invinciframes?
3) Is it normal that a parry must be executed before 2 consecutive spear R1s to work? Watching Jblackmel I've always thought that you needed to parry as the first hit struck, but it seems you actually have to parry earlier than that to parry the second hit.

If you fall from a ledge by taking damage and your Character is flailing his arms around

like this:

you won't get invincibility frames .

Don't really know what happened in your case though .

Nope, no arm flailing whatsoever.

Whatever it was, it looked glitchy as hell, lol. I was lying on the ground and then suddenly I warped into a standing stun position as the second bead hit, it skipped the 'getting up' animation entirely. I know this animation skip happens if you are hit by a killing blow while recovering from rolls sometimes but I've never seen it happen on knockdowns (much less, while flat on the ground, and not struck by a killing blow).

Does anyone else know what this is all about?

I never seen Dark Bead actually knock down someone, but this sounds like a serious case of lag. On the other end, you probably appeared to be standing in the same spot for all three Dark Beads to land, with your character not reacting to any of it. 
I would love to run some tests regarding weapon knockdown invincibility frames, but I need someone with a stable connection and it seems the PS3 version may be a bit laggier in general with things than 360 and PC.

Might it have, perhaps, been an intentional hack?

Unlikely, since this was Xbox 360, not PC. I mean I know 360 is famous for the 1:1 legit to hacker invasion ratio, but those are savegame modders and moveset glitchers. 

I've never seen something like this before.

Wait... come to think of it, I think this has happened to me before. I just passed it off and kept playing, though. Maybe that's why my post count is still so low-no compulsive posting >.>

With dark bead also, or a different spell/weapon?

Dark bead.