Let's break this down... and someone correct with if I'm wrong....
You buy 50 lowers at 5 lowers per 4473 so that's (10) 4473 forms. The dealer makes (1) phone call to NICS. Can a current FFL comment if the dealer is having them all approved at once, or does the dealer have to specify the customer needs 10 forms approved?
The dealer isn't telling anyone what the customer is buying over the phone. Why would the BATFE know you bought 50 specifically? How would they know what you bought? Did the dealer report you as being suspicious?
The customer probably shot off his mouth saying the lowers were an "investment" or something of that nature.
Once you tell your FFL some crap like that guess what, he reports you. They have to.