How could the buried corpses turn into zombies?

I've noticed that as well, on my first watching of the movie, but given that it was not that deep a movie, I've decided that it's a plot-inconsistency and nothing more.

The virus transfers by bite, scratch,... i.e., by blood. We've seen it being airborne at the start of the pandemic, in the Hive, but only then, because it was in concentrated form. We've never seen any proof that "the land has [...] been infected".

Even if we try to justify this by the "infected soil" theory, I think we introduce even bigger inconsistencies:

1. How did this happen? Surely, zombies didn't bite the dirt (at least not that much), and even if it did, a bit of saliva infecting the whole city's soil is... dumb. There was no mention of Umbrella doing anything like it, there was no obvious reason for them to do so, and no such symptoms were shown ever again.

2. The soil is full of dead things (worms, moles, bugs, buried pets,...). If the soil was indeed infected in some weird way, we'd be seeing an outpouring of dead insects and animals all the time. Just how many dogs would we have? And let's not forget cats, snakes, and other pets that must have been buried in the yards across the city.

3. Some of these corpses were just skeletons. How do you infect the blood of something that has no blood? How does a muscle-less skeleton move? I wouldn't accept a theory that the Umbrella Corporation was experimenting with reviving random skeletons in some cemetery.

4. Why did they rise exactly then, and how did they do so all at the exact same moment?

It was just to make another suspense/fight scene, nothing more. Most of the Resident Evils are fun action flicks, but I wouldn't read much into them.

This scene is a re-imagining of a similar scene from the game RE: Code Veronica (2000). And no, it doesn't make much sense. This inconsistency is even unintentionally covered over at SE Sci-Fi.

There are various fan theories, e.g. infected soil (which is already covered by Vedran Šego) and a theory suggesting that these people were infected by the virus before burial. No in-universe answer has been given, though.

Three possible answers, both as explained in the Darkside Chronicles regarding the Code Veronica graveyard zombies.

1. These were people infected with the T-Virus and buried alive. They turned and eventually broke free.

2. Rainwater penetration allowed the airborne T-Virus deep into the graveyard infecting the dead.

3. As we see in the rest of the series, the T-Virus doesn't just infect people. It infects animals, insects, plants, and the entire planet. It causes global ecological changes. The very Earth is dying and mutating from the infection.

Based on the games, the events at the Hive happen roughly 2 months before the full scale outbreak at Raccoon City. There, the outbreak happens because of zombies loose from the mansion (hive) in the woods around the city, as well as the water near Raccoon City being infected. It was not simply transmitted by bite.