Ninty needs to adress this, hopefully they'll put out a statement soon. I can think of several scenarios:
1. "Lol, no need to worry. Ye of little faith. Buying second hand will NEVER ban your console. We 100% gurantee it.
2. There are sadly a lot of counterfeit games going around - Here's a visual guide to EASILY spot fake cartridges.
3. After much consideration we've decided to put out a Switch² hotfix to unfuck our customers. In our ever vigilant fight against piracy we've put in place countermeasures that sadly turned out to harm our legitimate fans.
4. Fuck MIG, fuck used games and fuck you. Please understand.