Highly likely it's because the tower is being optimized. I've noticed this has become a huge thing for Sprint.
Now everything is back to normal and I'm back to an RSRP of -89.
I wish there was a way to capture the signal strength for several weeks and make a graph. Every time I call Sprint they always say that "there's no tower work going on with your tower, everything is fine". But I definitely lost 20dB of signal for about a week.
It's wise they are doing this considering how much more improved the experience is with a properly tuned antenna.