Cubs don't usually stay with the mom after they come out of the den their 2nd time together. If they did the only reason I could see was that the sow didn't come Into heat for some reason that spring. Sows have their cubs In January while denned up and the cubs again will den up with their mother the following winter. Come next spring around April or so the sow and her cubs come out again and when she starts coming Into heat she kicks the cubs out. If she didn't kick these 1.5 year old cubs away from her a boar that's wanting to breed a sow that's In heat will kill them. Sows generally get bred anywhere from May through July.