Several interpretations (some contradictory, some compatible with others) come to mind:
- Early miners didn't care much about then-worthless coins, and many of early coins are lost
- Early miners hoarded their coins as they didn't have to sell them to make up for the production cost; mining was dirt cheap (but so were the coins)
- Miners generally hoard about 30% of their coins, however with the advent of pooled mining block rewards had to be spent first (even if earned fraction is then hoarded by individual miners) - and this shows as drop in hoarding
- The nature of mining changed with the advent of SR: many newcomers set up mining operations specifically for SR purchases (freshly minted coins are significantly more anonymous then exchanged)
Any other thoughts?