You may be looking at transactions of an unknown entity dubbed "Crazy1o1". To my knowledge, it is still unknown who they are or why they are using 1-of-1 transactions, but they've been active for years. Notable mentions:
- LaurentMT first describing and naming Crazy1o1
- LaurentMT analyzing some traffic of Crazy1o1
- Stepan Snigirev asking about 1-of-1 transactions
- Crazy1o1 consolidating 720k+ UTXOs at over 100 s/vB while the mempool was essentially empty
- On-going traffic of 1-of-1 multisig transactions
- Crazy1o1 on OXT.me
Update: LaurentMT suggests that Crazy1o1 is a “Crypto and Forex broker named KOT4X”