I can't speak directly since I'm not investigating or privy to anything on that crime. All I will say is the gross redundancy of the same firearm for a single shooter is unheard of. About the only thing I could think of as a reason (if Paddock was acting alone) is he experienced operation malfunctions (jams, overheating, stock breaking, etc) when he presumably practiced with this particular type of weaponry.
If he truly was hellbent on ensuring maximum damage and mayhem perhaps he erred on the side of over compensating with a number of rifles loaded and ready, cycling thru them to prevent overheating and having redundancy should anything happen.
Still odd and unseen in my experience but at least it is a possibility.