Causes of Short Stubby Round Leaves?

[zxyqu]

Hopefully you know what I'm talking about, but I'm also posting a photo. It reminds me of an aborted leaf, like the plant started to throw a new leaf then decided... maybe not. I find it happens most often with species and primary crosses, so I'm posting it here. Anyway, I was wondering if anyone had any hard evidence it's caused by this or that? As far as I know, when they happen to me, there doesn't seem to be a glaring reason, but who knows. Maybe they're trying for special treatment, or upset I'm not paying enough attention...