If he were a pro, or at least someone with practice, he could have swapped mags rapidly. He wasn’t and he didn’t, thus the opportunity to jump him. The VT shooter took much longer to do his killing, but you are right in this case, much of it was over before most people comprehended what happened.
A lot of people at VT had a heads up and the opportunity to start mentally preparing themselves. Nobody in Tucson had such an opportunity chance...
...unless they are one of those rare persons who has given thought to their personal protection and perhaps even sought and received some training for it.
just the time to realize the situation and react would likely be too late to save the first mags worth of victims, especially when hes simply squeezin em off w/o much target choice...
Id be damned if Id just stand there and get shot tho...