Why does Yahoo take away some of the answers to my question?

Yahoo doesnt. The only way they are gone:  1. user/poster deleted them  2. glitch  3. reported by several users on here.

Answers get removed in 2 ways:
1 - the user who posted the answer chooses to delete it
2 - the answer was reported as a violation by enough other users, to get it removed

There are a few reasons this might happen:

1. The user deleted the answer before you saw it in the question

2. The answer was reported and removed before you saw it in the question

3. The answer is hung in the spam filter and only shows up for the user who posted it. It needs to be edited to get it to show up for others, but if the user doesn't realize no one else can see it, they don't know to inquire about the issue and find out what to do to get it through the filter.

Someone Reported them as a Violation of the Yahoo Answers Community Guidelines If they did not Break any Rule tho the Person who Posted them has a Right to Appeal them and if they do and If the Appeal is Approved they may come back later on 

Either, the answers were reported & deleted, or the person deleted their own answer.
There is also a spam filter here, sometimes when the user edits their answer, it will end up being ghosted (as in you can't see it) so it appears that it might have been deleted one way or the other.