I was thinking more along the lines of the NLB being in front of DNS servers and dropping resolvers

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