Apparently it was to show that there was still a life force in Superman and that he's not going to be dead-dead. Zack Snyder has the following to say in an interview with Entertainment Weekly. Sorry for it being so long but it's all kind of relevant.
This storyline is relatively faithful to the original Doomsday comic book history, since the monster also famously killed him in DC’s 1992 series “The Death of Superman.” Of course, Superman didn’t stay dead then.
The ending of BvS was designed to show that Superman was willing to make himself collateral damage if it meant saving others. I felt like we had to kill Superman in this movie in order for us to have been serious with the entire premise of the film,” Snyder says. “And that’s not to say that he clearly is gone forever.”
In the final shot, after Lois tosses dirt on the casket, we see some of the rocks rises up off the surface — a callback to the anti-gravity effect Superman had on the ground in the moment before he first flew in 2013’s Man of Steel.
There was another, more practical reason that Snyder decided to go this route, as well. He felt Justice League, which begins shooting April 11, would be a stronger story if it didn’t have Superman assembling the team.
“I wanted Bruce Wayne to build the Justice League,” Snyder tells EW. “I felt like with Superman around, it’s a different conversation when you create the Justice League, right? It’s like, ‘Me and Superman, we want to make a Justice League.’ [Other heroes would be] like, ‘Okay, yeah, I’ll join!’ I just feel like Bruce Wayne having to go out and find these seven samurai by himself, that’s a lot more interesting of a premise.”
It also forces Bruce Wayne to put his detective skills to work as he and Wonder Woman set out to find Aquaman, The Flash, Cyborg, the others. “I also I felt like, without Superman, there is definitely a vulnerability to the team that they’re gonna need to figure something out, you know? Superman does represent the powerful. He’s the Michael Jordan of heroes, he’s gonna score.”
With him out of the picture, the League can’t rely on his invulnerability either. And a part of the reason they team up is to find a way to bring Superman back — much like the goal of Star Wars: The Force Awakens was to rediscover Luke Skywalker.
Exactly how they’ll bring Superman back is still unclear. The rocks rising off the coffin was meant to suggest a lifeforce is still present, but it’s not simply a matter of sunlight rejuvenating him (as it does in an earlier sequence in BvS when Superman is wounded in a nuclear explosion in outer space.)
“He comes very close to death in space and the reason why we did that is because I wanted to show — and keep the idea in the viewer’s mind — that he can come pretty close to death and the sun can revive him, or he can be revived,” Snyder says, although after Doomsday, the Justice League is going to have to take more drastic measures of resuscitation. “I think something more is gonna need to be done.”
Entertainment Weekly, Batman v Superman ending: Zack Snyder explains what it means for Justice League