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](https://twitter.com/LaurentMT/status/1264586463536189444)
 - LaurentMT analyzing some [traffic of Crazy1o1](https://twitter.com/LaurentMT/status/1264958123275681793)
 - Stepan Snigirev [asking](https://twitter.com/StepanSnigirev/status/1413156991606865924) about 1-of-1 transactions 
 - [Crazy1o1 consolidating](https://twitter.com/ziggamon/status/1264233427840176129) 720k+ UTXOs at over 100 s/vB while the mempool was essentially empty
 - On-going traffic of [1-of-1 multisig transactions](https://mempool.observer/monitor/?filter=WD0xLW9mLTE%3D)
 - [Crazy1o1][1] on OXT.me


  [1]: https://oxt.me/entity/tiid/3498542669