Network effect, probably. One or two join, perhaps knowing why or because of their own circles outside the circle you're aware of. They know the other people (are still in touch from school) and encourage them to join, so you see a burst of activity among related (in a common fashion to you) people.

I've been watching the same thing happen over the past year or so as circles of friends from various periods in my life have joined. Often I see a burst of 3-5 people a month who are associated with each other in the same way they're associated with me (went to the same school, same school and major, hobby groups, bar trivia groups, etc.). It's normal, if you'd been on Facebook at the time it started you'd have seen something similar. The difference is that with Signal you get a prompt if you both have each other in your phone contacts, so you're notified when members of one of your social circles joins rather than having to discover them serendipitously (like with early FB, looking at friends of friends lists).