If their internal NLB monitoring can delete the A record for dynamodb that seems like a weird dependency (like, i can imagine the nlb going missing entirely can cause it to clean up via some weird orchestration, but this didn't sound like that).
Or an NLB could also be load balancing by managing DNS records--it's not really clear what a NLB means in this context
Or there was an overload condition because of the NLB malfunctioning that caused UDP traffic to get dropped
Obviously a lot of reading between the lines is required without a detailed RCA--hopefully they release more info