So far the most plausible theory is that Russia sabotaged the pipelines for two reasons [1]: 1) to claim force majeure and avoid massive penalties Gazprom would have to pay for the breach of contract (we are talking billions here; now they just blame the West and good luck proving in courts who actually did it); 2) to further blackmail and coerce the European nations (particularly Germany and Italy) who depend on Russian gas to stop supporting Ukraine.
As for explosions, they were recorded by the seismologist in Sweden and Denmark who are quite convinced it was due to a deliberate action: https://www.rferl.org/a/russia-gas-baltic-pipelines-leaks/32...
[1] https://www.forbes.com/sites/arielcohen/2022/09/29/russian-s...