The short answer to why Dimorphos now has a 10,000 km long tail, some two…
Here’s an article by National Geographic that sheds more light on the DART probe and its impact on Dimorphos orbit around Didymos.
The article contains an image taken by a European space craft right after impact. It shows debris tossed up in the sort of cloud-like pattern we would expect. However, later pictures taken by NASA show a star-like pattern. Later still, the pattern is that of a comet with a long tail estimated to be about 10,000 km long, or about 6,000 miles.
An article by NOIR Lab contains a detailed picture of the comet-like pattern. The tail is explained as caused by the Sun’s radiation pressure. That would be the solar wind, aka plasma current radiating away from the Sun. The tail is in other words pointing away from the Sun. However, the image in the picture shows a second tail, and there’s no explanation for it.
The amount of debris ejected by the impact of the DART probe is taken as proof that Dimorphos is a so-called rubble-pile asteroid. It has no solid core. It’s therefore a relatively low-density object that shed a lot of its mass as ejecta on impact. This explains the greater than expected change in Dimorphos’ orbit.
The new orbit is being monitored closely. The shape and stability of it is going to be studied, including the possibility that it may wobble. This means that there are other people than me expecting the orbit to partially restore, and it will be interesting to learn to what extent this happens, if it happens at all.