\key Moves \chords Below Staff

[Christian Eitner]

Hello,

I have difficulty understanding this tiny example, which is a
breakdown of my general problem:

{
%\key a \major

<<
        \chords
        {
                a1 c
        }
        \relative c''
        {
                a1 c
        }
>>
}

Without the \key command, the chords are shown above the staff. With
\key, they are shown below the staff.

I know from other threads that putting \key into a seperate block,
which encloses the melody, solves the problem, but I don't understand
why. Any help would be appreciated.

Thanks for your consideration,

Christian