I've been homebrewing for 17 years & I can't remember this ever happening. & it's with a yeast I'm using for the first time so I'm not sure if it could be a characteristic of the yeast. Wyeast 1332, Northwest. When I did a started I noticed it didn't kraeusen much even though obviously active. This seemed to be the case after pitching too.Fermentation started up right on schedule & continued at at staedy pace for a few days. Started to drop off after 5 or so, was quite slow at a week. Then at day eight I noticed the airlock filled with sludge. It had subsided but obviously some sort of vigorous fermentation had kicked in at about a week. Very odd. I had moved the bucket but I've done this many times before without having this happen. Actually, I do think I remember something like this happening the one time I tried my one & only little batch of apple cider. Anyone else ever have this happen?