My dad used to drive truck OTR. He used to talk about seeing lots of bikes on trailers headed to and from Sturgis around that timeframe. His opinion was that a lot of casual bikers would trailer until they were 50 mi away or so, park their vehicles, and then ride in like real hard-core bikers.
It's just an assumption from an observation, but this is the internet, so.......
We were on I-90 the first day of the rally this year. 4 hours worth anyway. Far more bikes in and on trailers than bring ridden.