A team that is "on the up and up" as I like to say, like that Cavs this year, the Celtics last year, the Nuggets and Blazers this year, the Hornets last year, is likely to cover the spread in a lot of games against weaker opponents.
The winning is new for them, the success, being the front runner, and that positive emotion feeds their bodies and energy. They are more focused, more consistent, and less emotionally. And I believe that a big factor in covering the spread is emotional factors and intensity.
Also, for a big spread versus a little spread, that might have to do with if the teams are really motivated or not. Just some thoughts.