there are ways to increase the amniotic fluid, but not sure about decreasing it.
have they looked at the kidneys well? is baby's bladder full? sometimes, with increased fluid, it could be a sign of the baby's kidneys not functioning well.
most other times, however, it's perfectly normal and there isn't a reason why.
I'm sorry you're being scared into losing your birth center birth! Is there anything your midwife is telling you about it?