And then I wondered how to express that in Lojban. My first thought was
{mu'i ma ta katna le jimca}, but I wasn't sure whether it was proper to
use {ta} for people.
{ko'a} is, presumably, wrong since it hasn't been bound yet (or would
it automatically be bound to the "nearest logical thing" or something
like that?). And {le va prenu} seems a bit long if it can be avoided.
(Hm, looking at the gi'uste, perhaps {ta ka'argau fi le jimca} is
better than {ta katna le jimca} when the subject is a person.)
{mu'i ma ta katna le jimca} does not link the motive specifically to
{ta} but it links it to the selbri. You observe that a branch is cut by
a cutting tool and ask for the motivation that causes this to happen.
So this sentence expresses the same question as {ma mukti lenu ta katka
le jimca kei zo'e}. And I think you did not really want to know about
the personal motives of the people who wield the cutting tools you are
pointing at (they might tell you that they really prefer uncut trees
but need the money), but you wanted to know about a more abstract
reason. You probably want to get an answer like {leka cumki daspo
nalseltro selfa'u le jimca}.