casette shifting

After riding this bike a couple of weeks with
no problem, shifting out, to smaller sprockets
on the casette have started to trouble,
sometimes nothing happens when I click.
If I understand the theory correctly, this is
because the cable is too tight, and you should
turn the barrel in (clockwise) to make it more
slack. This actually worked, but how can it
happen that the cable is too tight suddenly?
Shouldn't it be more loose rather?

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