Hi Anon, 

I think its pretty simple in regards tagging - if I’m not mistaken BPD officially stands for Borderline Personality Disorder however you’ll often see BiPolar Disorder shortened to BPD as well (I capitalized the ‘P’ in bipolar for understanding of why).

These post could also reference symptoms and / or feelings found in both bipolar and borderline or maybe cases of dual diagnosis, but my understanding of what you’re saying, it’s more likely just a confusion of proper initialism!