Simplest explanation is probably the correct one in the case. Ladd and Buff could best be described as the formal and informal leaders of the Jets. Both are without contract, and it appears more likely than not at this point that they'll both be elsewhere come 2016-17. While they're both pros, it would be naive to think that being in limbo wouldn't take some of the wind out of their sails. Even the vaguest sense that they don't have their hearts in it will likely create a ripple effect in the room.
I normally hate trying to dissect the psychology of pro sports (99% of it is just narrative), but I've felt since November that there's a sense of malaise with this team that just hasn't gone away. It's the same feeling I had with this team in the few months before Noel got fired.
The overall talent on this roster is not bad relative to the rest of the NHL; there's something more than lack of talent dragging this team down.