AsNoriuanswered grow question 1y ago I would reduce to : light , heat in flower, genetics, but its for healthy good grow !
Any missgrows could lead to it too . If you done everything right - only those 3 reasons are your options.
If you grew in temps higher than 27 Celsius - a lot of bud will be airy.
If you harvest early - you can loose density ...
Ph wrong - bad nuts uptake - airy bud
Weak light - airy bud ...
And so on ;))))
In other words - there could be many or main 3 ;)))