Likely it's just people that already have a gun, buying up more of them.
I'm certain that's some of it, it's certainly true for ME.
And the anti's are definitely working hard to sell the narrative of the "nuts" hoarding and doubling-down with ever more guns, while the number of individual owners is forever shrinking as a proportion of the population.
However, it's lunacy to try and explain away the massive deluge of NICS checks as being all existing gun owners. At a minimum, being conservative, I'd think it's reasonable to guess the swelling numbers of NICS transactions indicates the percentage of gun owners, or gun-owning households is remaining steady, if not slowly growing.
And taking into account secondary transactions, such as gifts, private sales, inheritance etc. there's going to be a non-trivial rate of redistributing firearms to new owners even if the initial NICS transaction was an "existing owner".