I'm a bit confused why you'd add kelp to the flush.....but everyone does things differently and it is what it is.
Were these fed organically or synthetically? With the mention of kelp, I'll ASSume you fed organically/with amendments/etc. There's no way to prove it, but if you were organic and gave them ample waterings...you may have just activated the build up of organics in your soil into solution and it whapped em. Most folks don't try to flush non-chem ferts. They just harvest em when ready.
happy trimming..... if there is such a thing