I'd guess that something broke off or killed the top of the tree (large animal, lightning, insect damage, freeze damage, who knows) and the tree sent out multiple sprouts in response to the damage. Several competing leads all became co-dominant.
epicormic sprouts as a result of 'topping', then because they were all about the same size, they all had the same amount of Auxins so they all competed successfully with each other. Not so unusual. Take a look at some of Gerry's pix of redwoods.
Most poplar trees around here are huge. Very fast growers they are. An hour north of me is the famous Joyce Kilmer Nat Forest. Its a stand alone grove of old growth poplar.Ive not been there yet but they are huge.http://www.main.nc.us/graham/hiking/joycekil.html
These trees can somewhat compete with the Giants of the west coast that Mr. B has lived in. They dont get very tall I guess max about 150 foot but they get very fat in DBH.
This is the Waslick poplar at Standing Indian in Franklin,NC. The pic doesnt do it justice I guess it would take 10 people to completely hug it. Its 8 feet in diameter.
They say their was a sister tree next to it however it was logged long time ago. The reason they didnt log waslick was because its sister tree had caused to many mules to die.