The fact that Nergal made a post about how he'll explain the delay later implies to me that whatever the cause of this delay is, it happened very last minute and it had to take priority over working on the build. Something personal happening (family member needs him, sudden need to work overtime on his real job, etc.) is my guess.
The problem with that assumption is that Omikuken (as is his habit) 'trolled' a post on Patreon about 7 hours before Nergal wrote his delay post,
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Only here to break your fun : it's not gonnna release on the 31st
So it seems to have been known in the team no later than friday morning (CET time) that 0.1.0 would not be delivered on time. It simply took 7 hours before Nergal wrote the formal delay post.
The circumstances of the why can only be guessed, as the explanation for the delay post also seems delayed here 24hrs later.
Personal takes precedence of course and always should, but given the current sitaution is an almost exact pattern repeat of last year, more than 5 lines saying delay and sometime in the future, could have used some more context.
That this post,
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RIP kinda glad I'm not supporting atm no offense.
Is the most liked currently is almost certainly a reflection of people seeing the pattern repeating and having waited it out on the side lines.
Given the developers personal sitaution (which hopefully will resolve favorably) and the history of AZL/UD, it is rather difficult to see how UD can be delivered timely, to say nothing of the Solarian project.
Not saying that to be impolite, but the matter of fact is, that there is a history over 1½ years now of various repeating issues (with amongst other things, taking too many projects on at the same time & reworking the games from scratch just before the goal line instead of finishing them) so it is a bit too generous to ascribe all the bumps/delays to random at this stage.
It is probably a major patience grind for its Patreons at this time.
Hopefully it will resolve to everyones satisfaction eventually.