All I can think of is either over fertilizing or not enough light.

That's all I can think of too...

Same as Cammomile, to much fertilser (particularly nitrogen which promotes leafy growth) or too much shade and they are reaching up for the light.

Are you planting them on a site that has previously grown legumes or you've sowed clover as a green manure. Certainly sounds like too much nitrogen as said above.

Maybe a bit more info on what you do would help, what manure, fertilizer etc you put down, are they planted under trees etc etc.

Thanks for the replies. I used to spread fairly fresh horse manure all over the raised beds in late October as there's always plenty up my local bike track. Then I'd leave it over Winter and dig it in in April, the following year. But I didn't do it last year, just spread a litle blood,fish and bone.
And they're in a position where they get plenty of light, being South facing.
Don't forget, I'm still a rank amateur at this game !