Anyone know the rules for this? It seems to happen when you take a few hits in a row, but it also seems pretty random.
I posted a similar question in the General Strategy section
That will probably be the one that best explains it all since I couldn’t find anything that expands on it further. This is for the Street Fighter game mechanics, just in case you meant a different series…
^ +1
Hello friends from the year 2009 we have this amazing thing on SRK called a search button. 
get friendly with it.
I don’t know how much of the old ST rules have been changed with HDR yet, and I don’t think anyone knows. I know that the thing that WAS supposed to be changed (but still seems in place, just to a less degree): Every single hit in ST pretty much has a small chance of dizzying you. Play long enough and you will find yourself raping someone for 30 seconds straight, getting jabbed, and then YOU eat a dizzy.
^This is less likely by FAR in HDR than vanilla ST in my experience, but I have no math to prove it.
-Also true of ST, and I have no idea about it’s trueness in HDR: At the beginning of a round, you are much more likely to be dizzied than as the round progresses.
-Each moves have a sort of dizzy “point system”, so certain moves do more damage to dizzies than others. For example, at the beginning of the round, if Chun Li immediately fireballs, and Balrog immediately does st. FP, they will both trade, with Rog doing more damage but Chun doing more “dizzy damage” to Rog, meaning if Rog is not careful for a few seconds, he will get dizzied hella easily.