Again, I'm speculating. I don't know motive, but you could easily see that doing so is intended to cause a disruption and sow fear amongst the population. Denying it also then obfuscates who it is and, just simply, adds more questions. This is part of the Russian narrative in the hybrid context, where it's below clear thresholds of conflict and the activities are simply intended to distract, sow fear, etc.
Maybe my colleague has a few other suggestions.