If there was a ‘who’ started the frenzy and run up, I’d say Rob Gough who paid a record $5.2MM for a ‘52 Topps Mick PSA 9 in January 2021 (
https://www.si.com/mlb/2021/01/14/mi...-breaks-record).
Then about 1 week later, one of the great last living HOFers Hank Aaron passed away. There was a lot of buzz in and about the card collecting industry and market at/after that time. Covid had been around for 1 year by then, and people realized it wasn't going to be the end of times, so didn't need to hoard their cash. Ongoing lockdown put more cash in people's pockets from government stimulus and savings from commuting, leaving more for discretionary spending.
The lockdown also put more time on people's hands, so had more opportunity to reconnect with or enter the hobby as an investment. Pre-Feb 2021 you could eBay best offer sellers on any vintage HOFer for a 15-25% discount. Now, you really need to be the highest bidder in an auction or take a chance on an upgraded card. Some cards pre-Feb 2021 included: 52 Topps Mick in PSA 1 for $15K. 48 Leaf Ruth in SGC A for $500. 53 Topps Mays in PSA 2 for $450. 52 Topps Mays in PSA 1 for $450 and $1600 in PSA 2. 09 T206 Cobb Red in PSA A for $1500. 51 Bowman Mays in PSA 1 for $1500. Most of these cards and others have doubled, tripled or better in past couple of years.