So is it fair to say that 'at lave' is one of those words whose meaning shifts in the passive? (I suppose it depends on context.)
Actually the translation is wrong. It should be "Why is it being fixed now?" or "Why is it being made now?". And it would still be a weird thing to say (the normal thing being "hvorfor bliver det lavet nu?"). The sentence holds no clue as to the more specific meaning of "lave". It could mean "fix", but who knows? "Lave" does not change its meaning in the passive any more than the average word does.
I don't think "done" is quite accurate here, although it would possibly work in some contexts. It would be more appropriate as a translation of "gøre". Fundamentally, "do" = "gøre" and "make" = "lave".