[jonathanstrange]

One reason to do this is to disrupt activities of foreign intelligence services, increase their paranoia, decrease their morale. The key is to do it randomly and not too often. That is under the assumption it's not a side-effect of surveillance tools, which would also be possible.

To give you a comparison, the East-German Stasi used to enter the homes of dissidents and do things like re-arranging towels, replacing fresh milk with rotten milk, etc. They also occasionally forced or convinced personal physicians to hand out the wrong medication to dissidents, worsening conditions of the patients or creating strange side-effects. They also radiated political detainees in prison to cause cancer. These were called Zersetzungsmaßnahmen.

I'd consider it credible that modern successors of the KGB like GRU and SRV, as well as related or allied agencies, could do something nefarious using technical equipment. That's why US agencies are investigating the reports, I guess.