Apparantly the canon reason for the crossguard is that it's not really a crossguard. The crystal Ren used to make his lightsaber is flawed/cracked, which causes it to be unstable (hence the wavering edge instead of a perfect beam), and the crossguard acts as a shunt for excess energy and the flaws in the crystal.
Not sure why he used a flawed one, but whatevs. It's Kylo Ren.