Falling backwards aborts our innate uprighting system. Repeated sitting back in a chair is the source of the infamous C-curve - the common slump.
The moment the pelvis and lower spine start moving backwards, we start to tense the neck and to flex the upper spine forwards. We do this in order to keep a level head. This is an automatic, survival-based reaction that allows us to maintain a functional presence in the world. The further back we fall, the more we need to flex the spine and tense the neck.
Also as we fall back, we need to tense the ilio-psoas muscles (powerful hip flexors) -- both to lower ourselves gradually toward the chair-back rather than letting ourselves slam into it, and also to pull ourselves off it once we have anchored ourselves there. Doing this repetitively over-works these muscles and strains the lumbar spine to which they are attached, making us susceptible to lower back injuries.