Basketball players, especially those who play in a caged court or indoor arena.
From 'cage' + '-er' (one who does something) + plural '-s'. In early basketball, courts were often enclosed with wire cages to keep the ball in play and protect spectators, giving rise to the nickname for basketball players.
Early basketball courts literally had wire cages around them to contain the wild bouncing ball—so 'cagers' is one of the few sports nicknames that comes from actual physical equipment rather than being metaphorical or insulting.
Complete word intelligence in one call. Free tier — 50 lookups/day.