Have you noticed them changing colors? What type of severum are they?
Normally, spawning severums will go into breeding dress. The bodies usually go dark, the eyes turn fluorescent, and the bellies of the fish will look white. If it's pre-spawning behavior, you should observe them cleaning the site they've chosen and randomly displaying for each other (flared fins and gills, swimming in front of each other, tail-slapping, etc.).