Why does bipolar send you spiraling into helpless sadness over insignificant things?

It's because of the way our brain chemicals are out of kilter. There are more or less three main ways an episode can get triggered:

Extreme stress/mourning/hearbreak/etc, which causes depressive chemicals to be released in everyone but is magnified tenfold by bipolar;

Mere chemical imbalance, which most often happens if an episode wasn't triggered in another way after a long period of time;

And then there's cases like you're talking about now, a blend of the two, which happens when your brain is starting to fall out of balance but isn't there yet, and even things that would normally be manageable can cause the chemicals to snap.

In other words, smaller things like this cause an episode because your brain was already gearing up for one, it just brought it on way before it should have naturally happened.

I hope this makes sense, sorry if it's a touch rambly <3