It's definitely a more recent trend though. I think load management has changed a lot. The season is just too long with too many back-to-backs to be overly concerned with seed. In the NBA I grew up watching there were usually 2-4 teams that you pretty much knew would be in the CFs and they rarely finished below 2. It's still sort of like that, but seems much more common that they end the season in 3rd or 4th than 1st, as well as low seeds moving past 1 round which was unheard of in the East for many years.
The Bucks finishing 3rd wouldn't have been a big deal, it was more that they just never looked good at any point. In a single game maybe, but they just never seemed to gel. There was something off about this team all year long and I think everyone was hoping they'd hit another gear later on, but it never happened.