The Industry is switching to majority digital distribution
When they relied mostly on physical sales stores stocked the albums on Monday's.
Since a lot of mom and pops and other record stores were closed on Sunday to give everyone the same release date they set releases on Tuesday.
Now that physicals are only being shipped to a hand full of stores (Target, Wal Mart & Best Buy) which are open on Sundays and do midnight deliverys they can have the albums fully stocked across the board on Mondays.
Plus Mondays are a good day for ecommerce being that a lot of sales happen during the work day.