I was at the same place and wondered the same thing -- saw them the first time on Friday, June 25, at the bend in the H10, just before the dirt road turns right, behind Muntshe mountain. The zebras were in the black, burnt section across the Mlondozi river.
I did go back the following Monday on the long s128 loop, and found many paths criss-crossing the road, which had recently been burnt black. I liked the patterns, and hoped to find some zebra crossing the road, which could make good photos -- black and white against black and yellow paths.
The next day it rained, but in the evening I went back there. I found a few zebras disappearing in a northerly direction at sunset. Did the same thing again on Wednesday morning, and saw lots of zebras at a waterhole where the s128 meets up with the H10. Had breadkfast at Nkumbe lookout, and saw a huge herd of zebra moving south, towards the s128.
I immediately stopped eating and drove back to the s128. About 3 km down this road, I met up with a few hundred zebras, slowly plodding along across the road in their DAILY? mini-migration in the direction of Muntshe waterhole, where I had seen them the previous Friday. I did get some interesting pictures, but the rain had washed away a lot of the black cinders and soot, so not as dramatic as hoped for. Will put one or two of these pics on my trip report within a day or two: viewtopic.php?f=27&t=45138
The only conclusion I can reach is that these animals do a daily am walk to the waterhole near the H10, and return to grazing grounds near Nkumbe in the afternoon.
Of course there are bigger migrations, and Mlondozi area is known for winter concentrations of huge herds of zebras.
In the 60's my dad took us to a similar spectacle just east of Tshokwane near the Wolhuter tree, and there were thousands of wildebeest congregated there during a dry winter. It seems as if many of them have died in the meantime, and the numbers haven't recovered (change of habitat?/increase in lion population?).
Lets wait till some boffin can give us the bigger picture!
Friedrich von Hörsten