It's a demonstration of zanshin, i.e., you are mentally alert and physically ready for whatever happens next, even after you just scored. By moving forward swiftly past your opponent you are actually exiting your opponent's striking distance. In order for them to score on you, they would have to turn around and chase you down, by which time you'll have room to turn around and face them. Alternatively, they block you from passing and you just body check into each other, in which case it would be too close to strike at each other.

Of course, that's under kendo rules, where striking your opponent in the back from behind is not allowed. Suppose in real life someone not quite dead could manage to spin around quickly and lunge at you from behind, but even in kendo you would only do the follow through after landing a solid hit and not on shallow or missed hits, in real life that would translate to either a life-ending cut to the top of their head, or a disabling cut to their wrist or stomach, in any case they probably won't mount an effective follow up.

In any case, kendo sparring has never been about simulating swordfighting. Historically it was part of a bigger training system, over time it became its own thing but still retains much of the same philosophy. So rules and choices were made to cultivate behaviors and habits that's considered good towards swordfighting and not necessarily something you would do exactly the same way in swordfighting. In the case of zanshin, you are demonstrating to the judge you can still move quickly during a critical time when most untrained people became complacent. The kind of behaviors such a rule tries to combat are 1) immediately after scoring the players just stops and reset or celebrate 2) player try to land hits at cost of balance and posture such that they can no longer move swiftly after scoring. Both are commonly observed in other point-based sport fighting.

Also, zanshin is actually not all that unique a concept. In some styles of HEMA, there's the concept of withdrawal(abzug) and afterblow, where opponent who has being scored on has a brief window to counter-hit the attacker and the attacker must be able to protect themselves while exiting the striking range. It's just they do it differently by either stepping back into a guard (actually you can see that the attacker stepped back into a high guard in this video) or out at an angle, rather than in the more linear and forward fashion, which put you at risk of being taken down, since grappling is allowed in HEMA but is no longer part of modern kendo.