Of course the attempted murder of congresswoman Gifford and the very real murder of the six people who did die is not an act of political violence, let alone terrorism white rightwing violence is always exceptional, an unfortunate incident, the act of a mentally ill person with no need to be put into context:

It is a common phenomenon, long studied and explained by social psychology that when individuals from our in-group or privileged individuals commit questionable acts, these acts are explained individually. When individuals from out-groups, or groups that are socially unpopular, commit questionable acts, these acts are explained as part of group membership, as categorical. The former are exceptions, the latter are representative. That is how racial and ethnic prejudice persist and how white privilege is preserved. One only has to imagine what media discourse would be, had the shooter been non-White, Latino or Muslim.

So, this timeline is one of white, domestic terrorism, fueled by seditious rhetoric from various media outlets and figures. The fact that the perpetrators are not part of a single organization does not change that fact. Social movements can exist without social movement organizations.

As for mental illness, it is the easy individual culprit, the one factor that, without further elaboration, explains everything. This is as if mental illness existed in a social and cultural vacuum, which it does not, of course. To invoke mental illness may explain outrageous behavior (i.e. behavior outside of the norms) but it does not explain the commission of specific acts (assassination attempt against a Democratic Congresswoman, that still remains to be explained). But to invoke mental illness provides some relief that we are not dealing with organized violence and that therefore, there is nothing socially and politically significant going on and no deep questioning to be had.