I think it's because the plant is crowded. These plants have more than one growth point and all are competing for everything. Dividing them gives each part more of everything they need.

given enough time.. many plants will fairy ring.. the best growth being on the exterior where it is growing into fresh soil .. and the middle dying out ...

at that point the old and dead needs to be removed...

and by doing so.. you add fresh new soil ... air and water circulation .. there is just no downside ...

though we think of them as perennials.. any given part of the plant does not live forever... but the plant does ...

as pieces mature.. and head toward the geriatric years.. they simply lose vigor ... just like grandpa.. lol ...

i really think its just the fresh un-compacted soil ... plus teenage plants ... that simply explode with vigor ...

not much science there.. just observation and opinion ...

ken

Last spring I got impatient about my salvia argentea coming up thru the bark and I got to poking around in the dirt below and came up with a teeny toothpick thing with a bit of green on it. In a panic I thought oh no what have I done, pulled up my salvia???? I plopped it back in the dirt 15 inches away and patted soil over it...along with bark and... then just hoped. By August I had two beautiful salvias with big gray fuzzy leaves! Now if I did that on purpose (like I'm tempted to) I would probably come up with nothing. I just don't know what I'm doing.