I think I can offer an explanation if ya'll don't mind, and keep in mind that this is just my own personal take on the subject. I think you guys have heard me rant on the corporate takeover of our society before, and may be getting tired of it. Yes, I blame all our modern problems on that one thing so it starts to look like an obsession. But, if you don't want to bear with me, then you don't have to read it.
Short and sweet - In a society run by corporations, society starts to look, act and even think like a corporation. Corporations think only in terms of commodities - assets and liabilities. (Yes, I used to be an accounting major, among a dozen others.) People are also depersonalized. Everybody (what they call "labor") becomes a commodity, either an asset (good for the company or society) or a liability (something that is subtracted from assets to give us capital). If you can dispose of your liabilities, then capital benefits.
The problem is, if you apply the "accounting equation" to people, then some people start to be seen as assets to be used or liabilities to be disposed of. Some people call this "social Darwinism." In human society this type of corporate thinking is called (as Mick pointed out) sociopathic, but in the corporate world it is seen as normal.
(You guys keepin' up with me?)
So, what we have is people often times (because of the corporatization of civic society) who aren't thinking of people as people any more, but as objects (commodities) that are to be used, which is often the very definition of a sociopath. Let's now say that a person is seen as not only a liability but an actual obstruction to the things that this individual wants. The person is no longer a person in that individuals eyes, so it is OK in his eyes to use violence to either get that person's cooperation or eliminate that person altogether. In their corporate-addled mind, they are simply liquidating a liability.
In addition to this, some get really resentful of having to be seen day in and day out all their lives as commodities. They never know when that day will come when they will be flipped over to the liability side of the equation and **POOF** they get laid off or fired. All that stress has to be building up in them and is just waiting for a trigger. When that trigger comes ... well, that's where you get workplace violence.
But then again, some people are just loony-tunes, like that kid at Virginia Tech.
Well, there's my theory, and you can just decide for yourself whether I've got a point or whether I'm just full of sh**!