Yes there are increased risks associated with riding a lift while wearing a pack
The first major risk, mentioned by Paparazzi, is hangups. These occur anytime the pack becomes entangled in the chair. These occur fairly frequently when unloading especially with chair riders unfamiliar with the hazard. The danger is compounded with inattentive lift operators as the likelihood of going "around the wheel" and being exposed to real danger goes up.
The second major risk is a slip off the seat. Since the backpack occupies horizontal space the rider has less area to sit and the riders weight is centered much closer to the lip of the chair. My ski area uses deeper chairs to mitigate the issue. Slips tend to occur mostly during loading.
A common time for a generalized slip which does not depend on packs occurs when the bar is moved upwards in anticipation for unloading. This risk for this type of slip increases with the use of a pack.
As Pont pointed out in the comments a pack will push you closer to the swing space of the safety bar. This makes bar strikes more likely if when you aren't paying attention and/or your fellow riders don't provide a warning they are moving the bar.
While there are increased risks packs also reduce risk. The can provide fall protection, additional warmth, and obviously carry supplies which can reduce risk depending on what you bring and how you use it. While they are a hassle for lift operators, ski areas have not banned them likely because the real risk of injury or death combined with lift operator frustration is far less important than lost skier dollars to those who prefer to carry their lunch, water, avalanche gear etc).
At the ski area I frequented for several years backpacks were common since they were mandated and lightly enforced for the hike-able terrain (which accounted for much of the ski areas draw).