Here is a large statistical study that claims a correlation between time spent sitting and mortality rate that exists independently of the well known correlations between level of physical activity and mortality rate. More exercise means a longer life expectancy while more sitting means a lower life expectancy even after considering that those who sit more often are most likely exercising less.
We linked prospective questionnaire data from 222,497 individuals 45 years or older from the 45 and Up Study to mortality data from the New South Wales Registry of Births, Deaths, and Marriages (Australia) from February 1, 2006, through December 31, 2010. Cox proportional hazards models examined all-cause mortality in relation to sitting time, adjusting for potential confounders that included sex, age, education, urban/rural residence, physical activity, body mass index, smoking status, self-rated health, and disability.
The association between sitting and all-cause mortality appeared consistent across the sexes, age groups, body mass index categories, and physical activity levels and across healthy participants compared with participants with preexisting cardiovascular disease or diabetes mellitus. ... Prolonged sitting is a risk factor for all-cause mortality, independent of physical activity.
This is not the first study to propose this correlation, of course. There are a range of others from past years. One has to wonder what the mechanism is here, however - my suspicion is that it actually does all come back down to the level of physical activity in the end. In these massive studies the level of exercise and activity is reported by the participants. A person who stands and works is going to be somewhat more active than a person who sits and works, even though that time may not be categorized as physical activity, or reported differently.
Exercise is much like calorie restriction - the effects are so large in comparison to other factors we have easy access to that they are likely to creep into any study.
You might look at a recent study on activity and Alzheimer's disease that was one of the few to use measuring devices rather than reports of activity. One point that emerges is that a fair degree of ongoing low level activity and exercise won't be classified as such by the participants of study without machine measurement. Housework, taking out the trash, the small increase in energy expenditure from standing while waiting versus sitting while waiting, that sort of thing repeated day in and day out. How much you are sitting really does sound a lot like a proxy for how much activity you are undertaking when you are doing things that most people don't really count as activity.