BPD and BP, especially the rapid-cycling variety, are well-known to have such a significant overlap that distinguishing the two has confounded the medical community for quite sometime.

Often, something of the sort--"Well Borderline is more triggered--not so in Bipolar."

While I don't have a PhD or an MD, I'd say--anecdotally--that this is not the case. A depressive episode can indeed start as irritability, as can a manic episode.
In these stages, given irritability, a person may seem more responsive in a "BPD" fashion.

What I think distinguishes the two is that the Bipolar episodes will continue along their trajectory--i.e. an irritability due to depression will devolve into sluggishness and potential suicidal ideation.

I've been diagnosed with both, although most have said I just have "borderline traits." I notice these are very reactive and don't have the "steady" trajectory that I mentioned above. It's far more haphazard.

So it's not reactivity as much as it is a far more random trajectory over time.