Long-term holders (LTH) always sell older coins as BTC prices strengthen in a bull market.
It is natural for LTH to sell part of their stack to finally buy their dream house, dream car, or do something meaningful to improve their quality of life.
Since these are older coins, bought "on the cheap", they are most likely selling only a small portion of their total BTC holdings.
The supply of coins in the possession of LTH will most likely continue to drop as we move further into the BTC bull run.
When we see LTH increasing their stack, though, that will probably indicate the end of the bull market.