The league got better.
Few teams starting to show the fruits of tanking.
I think the league, especially the East, being overall better has a lot to do with it. Look at the teams that they've lost to recently: Denver, Houston, Detroit, Miami, Boston, OKC, etc. (not counting LAC or Chicago when they rested several guys). They can't sleepwalk through the regular season and still get the number one seed anymore like they did in years past.
These teams are either 1) really good and close to the Cavs' overall level (Houston, Boston), 2) pretty good with significant matchup advantages over the Cavs (Denver, Detroit, OKC, Miami), or 3) playing their best ball of the season so far (Miami).
They'll turn it up and be a bit better during the playoffs and should still be considered the favorite in the East, but I don't think they're overall as good as last year. People don't realize how old many of their rotation players really are, along with some of their younger guys having significant injury histories that have slowed them down and/or significant miles on their legs already.
Depending on the matchups, they're going to be really challenged in the East this year. If they get a Milwaukee/Miami first round, Toronto second round, and Boston third round, then that's going to present a daunting task for Cleveland that will really challenge them, unlike the last several years where they've waltzed through the East.