I get this too on occasion, sometimes randomly and sometimes if my mood becomes really negative and dysphoric, I might explode in the opposite direction for a few hours and sometimes weakly for a few days.

I was mistakenly diagnosed bipolar 1 because my rapid and severe stress related mood swings were interpreted as a mixed episode by a doctor who didn't even observe them.
I think this has little or nothing to do with actual hypo/mania. I've never gotten the impression it's a random chemical imbalance and this is supported by the fact that I don't respond at all to bipolar mood stabilizers or anti psychotics.

Although I'm not diagnosed BPD, I believe borderlines lack certain emotional and cognitive breaks, meaning you if start to feel good or bad, your brain may exaggerate this and you end up inappropriately happy or sad, or whatever combination of emotions and feelings. An inpatient doctor once said I have mood disinhbition and described it like that.