So I am brewing lots of bitters using WLP002. Typically, I mash a grist with around 10-15% specialty malts at 66 C and ferment at 20 C. (I use BIAB and dunk sparging.) The typical situation is that after 2 weeks the SG is around 1.013 with all activity stopped, a huge cake of cheesy WLP002 on the bottom of the fermenter. Samples taste great.

Then, I bottle the beer, and sometimes fermentation kicks in again, the finished beer reaching SG 1.008, or even lower! Needless to say, I am not too happy about these over-carbonated and dry beers. 

These bitters are probably not infected, I am very meticulous in my process, and it is only 002 that behaves like this for me.

Why is this? The closest plausible explanation I have is that the usual low attenuation is due to extreme flocculation. Rousing the yeast just a little bit at bottling time starts the fermentation again.