Assuming the explosions are relevant, some possible causes, ordered somewhat (but not entirely) by my personal estimation of likelihood:
* Ecoterrorists -- I discount this mostly on the basis that they would have claimed responsibility by now.
* NATO sabotage -- this reduces strategic options for several EU countries (albeit a strategic option that isn't really being exercised and is almost universally disfavored).
* Russia sabotage -- this isn't terribly rational for Russia, but Russia has been unwilling to pursue rational options for a while. I can see someone in Russia thinking this will stoke anger in Germany (and the rest of Europe) that may create European pressure to force Ukraine to end the war (which is the most viable path I see at this point for Russia winning the war).
* Ukrainian sabotage -- pretty much the only actor for whom sabotage makes rational sense. Especially if it is a demonstration of Ukrainian cyberwarfare capabilities.
* Poor maintenance -- this strikes me as unlikely because NS1 is quite new infrastructure (2011), so it would have to be extremely substandard to have failed this quickly, and I believe most of the construction was done by European and not Russian firms, so I don't think that's at all plausible. But there may be some failure modes I'm not thinking of here.
* Ecoterrorists -- I discount this mostly on the basis that they would have claimed responsibility by now.
* NATO sabotage -- this reduces strategic options for several EU countries (albeit a strategic option that isn't really being exercised and is almost universally disfavored).
* Russia sabotage -- this isn't terribly rational for Russia, but Russia has been unwilling to pursue rational options for a while. I can see someone in Russia thinking this will stoke anger in Germany (and the rest of Europe) that may create European pressure to force Ukraine to end the war (which is the most viable path I see at this point for Russia winning the war).
* Ukrainian sabotage -- pretty much the only actor for whom sabotage makes rational sense. Especially if it is a demonstration of Ukrainian cyberwarfare capabilities.
* Poor maintenance -- this strikes me as unlikely because NS1 is quite new infrastructure (2011), so it would have to be extremely substandard to have failed this quickly, and I believe most of the construction was done by European and not Russian firms, so I don't think that's at all plausible. But there may be some failure modes I'm not thinking of here.