It could be: too much water; too rich soil; too much fertilizer; bad cultivar... or all four. But it's usually one or all of the first three.

Where were they planted?

And what herbs were they?

Two things come to mind. The first is a CA Kate said, too rich a soil for drought tolerant plants like sage, oregano and thyme. The second thing may be very basic, but if you are new to growing your own herbs it's good to know: some herbs get more intense when they are dried.