Why did my bubble drop early?

Shields drop during PVP and will not protect you from PVE.

Your shields also drop if you cross the threshold near the level 20 bases area, however a message is given that it will happen. Also, shields drop after a set time, normally 7 days at the start of a world or an hour if a 2nd or subsequent base.

I wasn't anywhere close to level 20 I only crossed over the sector line. Barely

In which case it must be a bug.

no pvp either.

I think the exact border for the Protection bubble is influenced on the world size, so by crossing that sector line you might have slipped right into that range.

Where did you place the base and did you move it?

I'm investigating now after I am back in the office. It seems that bases founded beyond level 18 and higher may indeed get targeted by Forgotten. I'll let you know if I know more.