You often see spirochetes when there are "septic" conditions.
Septicity is produced by anoxic or anaerobic conditions allowing for fermentative respiration. During fermentative resipiration, bacteria produce organic acids as the end product of metabolism.
Spirochetes may also be common with influents containing organic acids (common in brewing, fruit and vegetable processing, winery, etc).
No matter what the source, high levels of organic acids can cause problems with filaments that thrive on the low dissolved oxygen and readily soluble substrate. This is why systems with high influent organic acid loadings often see fewer filament problems with a complete mix reactor or step-feed in plug flow reactors. Another source of organic acids is a long residence time in the primary clarifier or upstream lift stations.