Gazprom unilaterally cut off gas supplies at the direction of the Kremlin, "weaponizing energy supplies" to Europe.
At some point, if Gazprom wanted to come back to European market - they would be first greeted by billions of dollars of contract charges in arbitration courts.
(and I guess the number of billions is probably in the 10's or more)
Therefore, to avoid that fate, Gazprom or the Kremlin surreptitiously blew up Nordstream2 themselves, in order to be able, later, to claim in court that the could not have resumed gas deliveries if they wanted to. This would be an argument against the billions in contract charges. Basically, they incur the cost of blowing up (and later repairing, one presumes) their own pipeline in order to avoid the cost of the fines and legal sanctions for suspending gas delivery unilaterally.
Summarized as: the Kremlin miscalculated in suspending gas delivery, and by blowing up the pipeline is trying to preserve some future access to the European market, after current hostilities cease.