This weekend was the Winni Derby Salmon/Lake Trout event so there were probably more fisherman than you are used to seeing which might explain why you had never noticed this before.
Fisherman are trolling (moving slowly with bait behind them) for salmon and lake trout and many use any of a number of brand name planer boards such as this:
https://www.bigjon.com/product/otter-boat to allow them to attach leadcore (weighted line) to spring loaded clips on the line these planer boards are attached to. These clips slide down the line toward the boards and allow fisherman to spread on 2 or more leadcore lines with bait on each side of the boat without tangling with each other.
Many will have these boards 30-50 feet off the sides of their boats so be careful if passing closely.