They've been using all sorts of tactics to lower the pressure of gas delivered through the pipes. They claimed technical difficulties, then stopped the flow for a while. Then they restarted it again.
But they were bound by a contract to keep delivering to those countries that caved in and accepted those payment conditions of making the conversion to rubles. If they didn't honour the contract, they'd have to pay some penalties.
So then they decided that they couldn't use technical issues too many times as an excuse to keep harrassing Euro clients by creating artificial scarcity, thus driving up spot market prices.
That's why they decided to torpedo some parts of the pipeline, leaving out only one, to keep increasing the pressure on Euro countries to stop sending weapons to Ukraine.
It's a move that was supposed to increase pressure on Western countries to stop supporting Ukraine by both withholding gas from them and by doing that also increasing gas prices in the market.
They left one pipe intact just in case Euros change their minds and stop supporting Ukraine for the sake of more cheaper gas, so they could deliver through that pipe if it came to that.
This scenario fits well with how the Russian leadership tends to think about solving problems through brute force and blackmailing the enemy with a known weakness.
or the Kosova bombing or 20 year retaliation against a country which ultimately had very little real connection to the terrorist attacks. Continuing to supply weapons which are used against civilian population for years now.
