Between this and the coyness of most European countries governments at the time to comment on investigation, it's not too far fetch to think that Ukraine might be involved.
I believe they are only wondering about details now and trying to hear the guys.
https://www.wsj.com/world/europe/nord-stream-pipeline-explos...
https://www.intelligenceonline.com/government-intelligence/2...
https://www.france24.com/fr/europe/20240814-sabotage-de-nord...
Who did benefit most from the north stream sabotage?
At the end of the day -- nobody of course, as the whole idea that the sabotage could bring any significant strategic benefit (even in terms of the "psychological" front) was pretty much braindead from the start. Meanwhile all it seems to have brought to the table was added instability, more paranoid thinking all around (in addition to the quite substantial methane release).
But it's definitely easy to see why (at least some of) the Ukrainians thought their side could gain something from of it; or they may not have been thinking in terms of any specific strategic advantage, but rather simply spite.
Either way -- the decision was made, and the job was done.
A coup from disaffected underlings unhappy with the status of the Ukraine war is hardly a weird theory. He's fought off one already!
Russian involvement is a bit far fetched to me. It severely limited their ability to export at a good price when gaz sell is how they finance the war in the first place, and removed their main pressure point on Europe therefore making the war considerably harder to win.
A third party would be more likely (there is a long list: could be the USA, a European country which wants the block to align strongly with the USA, or another power benefiting).