I'd not heard previously of this being the case, but if it is then the reason is as follows.
You have two competing effects. The polarity of the HX and the strength of the HX bond. A more polar molecule is better at attracting the alkene's pi bond electrons. In decreasing order the polarity is HF, HCl, HBr and HI. However this is also the bond strength order, and we need to break the HX bond, so weaker is better. Hence HBr represents the compromise between HX having enough polarity but also a bond that isn't too hard to break.