They were simply walking through the storm. If storm was strong enough to move the MAV, humans who are a lot lighter should've just flown away, no?
The Real Answer, as from Andy Weir himself: There isn't. While the book is grounded in science, there are no such storms of record that could create the effects that are shown in the book or movie. It is a large, windy, atmospheric MacGuffin. The storm is solely there to move the crew off the planet, leaving a presumed-dead Mark Watney to a disco-themed purgatory. (I'd link to the Triangulation podcast episode where he talks about that, but don't have access atm. I'm sure he says it elsewhere.)
The in-universe answer: All of the landing sites are in craters, for the sole reason to mitigate impact of wind. The MAV is the highest point of the landing site, it's large enough to be caught by the wind, but with a small enough base to actually be able to be tipped over. That is why there is a decided plan on how to react to windstorms of various magnitudes already in place while they were in the HAB.
Due to ground friction, wind speed close to ground is slower than at higher altitude. That's why wind turbines are so tall. Wind speed at the height of the MAV should be a lot higher, because of that.
The MAV had a higher surface area exposed to the storm and therefore was exposed to more force. It may have been top heavy and is less able to correct it's balance against the storm. A human can lean into the wind and shift around to find a balance point.