I've just had an answer automatically converted to a comment at When is the $lcm$ of a fraction sum the actual denominator.. How does the system detect trivial answers? (So I can avoid the automatic conversion...)
The main trigger is the link to another answer on the site. As Jeff Atwood explained on Meta.SO
This only applies to answers that consist of a bare link to other Stack Overflow / Stack Exchange questions with very little other descriptive text.
The idea behind it is, that if the question can be answered simply by posting a link to another question on the site, it very likely is a duplicate anyway and you should have voted to close as duplicate instead of answering.
So, to avoid this you just need to add some more explanation to your answer and avoid posting a link without further elaboration.