If You Overwater Them, It's like Mudwrestling

I received the following e-mail from a client today.

Also i have moticed 2 penstemons which were planted too close to other plants and now that they are growing in they are fighting each other so need to move them over a bit.

It sounds like sibling rivalry (my brothers and I didn't always get along), but I've never heard of plants fighting (even the ones with thorns).  It's breezy today; maybe the wind is blowing the leaves of one plant against the leaves of another.