Why are blocking artifacts serious when there is fast motion in MPEG?
DCT is both discrete and finite, it has a limited range in both the space and frequency domains. If the change in a certain location is large or the location changes a lot, the results of the transform will be beyond the sampling range in the frequency domain.

Since as you mentioned MPEG is based on blocks, the result of a block with with missing high frequencies, after transform back to the spacial domain, will result in the loss of detail in that block - making the block look flat.

For a more accurate result try Signal Processing Stack Exchange.