"He who can destroy a thing, controls a thing."

The Russians have seen the loss, (or at least, the severe degradation) of their ability to supply (or evacuate) Crimea. They can still use the "land bridge", but that is now threatened by Ukrainian offensives in the North. A system of ferries could do the job, but it would be cumbersome and expensive, and also vulnerable to Ukrainian attack. The loss of this bridge, especially the day after Putin's birthday, and in the aftermath of the referendum and the annexation, is especially humiliating.

It is my understanding that the total destruction of this structure by aerial or artillery attack is very difficult, and the Russians have a formidable air defense there. But any attempt to repair the structure WILL be vulnerable to drones or missile raids because it will involve moving construction crews and equipment to the site of the breach, and they are vulnerable to attack.

I have a feeling this action has been contemplated for a long time, but the Ukrainians have delayed it in order to maximize its psychological effect. Supposedly, there are now 50,000 Russian tourists stranded on Crimea right now, and the occupying authorities are reassuring them that they can still get home safely. Perhaps Putin can organize a Dunkirk-style evacuation. Meanwhile, Ukrainian postal authorities have announced they are designing a special stamp to commemorate the event.