This needs no 3D.
There's static distant background layer (sky, cliffs). Then there's the moving background layer. My first idea "it's a big perspective drawing simply zoomed out gradually", was wrong. Testing it revealed that the effect looks out sloppy. There must be more.
An idea of 3D modelling with flat objects in different distances still looks out an overkill altough it's not impossible and definitely easier than full 3D.
In 2D one can use one point perspective transform which is stretched inversely. In the image the border between the sky and the grass isn't in infinity, it's quite near. It can be thought caused by curved earth, more likely a top of a ridge, maybe one mile away.
The grass can be thought as a road which is apparently wider than the image at the max. visible range. If the apparent horizon line was in infinity, even the largest grass field would be there a dimensionless (vanishing) point.
So, in the beginning of the episode there's the grassfield stretched with perspective transformation. A certain distance is at the apparent horizon line (=top of the ridge). The static background layer covers everything above the apparent horizon line.
Frame by frame the stretching is released, but keeping whole time the same distance at the apparent horizon line. There's 3 more snapshots:
Unfortunately I haven't advanced video software where the gradual perspective destretching could be programmed.
The grass obviously must be defined as vectors or deformation this heavy spoils the sharpness. Aternatively there can be massively oversized pixel dimensions.