I don't think it was incompetent attempts to attach something to the lines. It severed 4 data data cables and electricity cable called Estlink 2. If it hadn't been stopped, it would have severed also Estlink 1 within half and hour later perhaps also Balticconnector gas pipe.

About year ago Balticconnector gas pipe was already damaged by a Chinese ship, which had been dragging the anchor for long time in a very suspicious manner:

https://en.wikipedia.org/wiki/Balticconnector

And again in August several data cables were severed by a Chinese ship. According to one analysis, that time they may have dragged the anchor for 400 kilometers:

https://www.youtube.com/watch?v=fB-vEp3wr-0

BTW, Russia seems to have for years spend time finding out the routes of sea cables and perhaps also spied them or prepared some kind of sabotage:

The global internet is powered by vast undersea cables. But they're vulnerable. https://edition.cnn.com/2019/07/25/asia/internet-undersea-ca...

Ukraine war: The Russian ships accused of North Sea sabotage https://www.bbc.com/news/world-europe-65309687

Once is happenstance. Twice is coincidence. Three times is enemy action...