Heavy fresh manure may cause forking too

Heavy fresh manure may cause forking too

Or transplanting the seedlings. Break the tap root that these plants produce first and they will then grow new ones thus causing a fork.

You need a very fine, sandy, tilth to grow tap rooted veg including carrots and parsnips. Otherwise if the root hits the tiniest stone it will bifurcate. So a thin sandy/loamy soil free of all stones.

Always found that odd about the fine sandy soil. Ours here is as gravelly as you can imagine (bottom of a lake once upon a millennium) and yet we manage good long straight roots on Parsnips.
Carrots don't grow. Root fly destroys them no matter what we do, even in the Polytunnel.