RFERL now has a fairly lengthy article on the incident, which seems to point more in the direction of local power struggle... and reciprocal accusations between the mayor and the governors' office as to whom actually set the fire...:
Russia’s Investigative Committee said on April 29 that it had launched a preliminary investigation into a fire that occurred a day earlier near a forest on the outskirts of Ust-Kut, a Siberian town located 850 kilometers north of Irkutsk.
The committee said a local district official, who was noticed at the site, may face a charge of abuse of power.
No names or other details were provided by the committee, but Ust-Kut Mayor Aleksandr Dushin told RFE/RL that he and his associates had caught four district officials, including Deputy District Governor Mikhail Bars and the district governor's press secretary, Yekaterina Anisimova, as they -- or so Dushin alleges -- set dry grass on fire near a forest not far from Dushin's private lands.
The Ust-Kut mayor insists the four also were filming the fire using a drone, in order to accuse him later of being an arsonist.
Dushin provided RFE/RL with several videos, in one of which he is seen checking the contents of three plastic canisters aboard a boat allegedly used by the district officials to travel to the area.
The mayor says on the video that the canisters are filled with gasoline as a visibly beaten man, who was later identified as Bars, sits slumped across from him.
Ust-Kut district Governor Tamara Klimina, meanwhile, rejected Dushin's accusations, saying she had sent her deputy Bars and others to check reports about fire cells in the forest at three sites.
One of the sites was close to Dushin's private lands, where Klimina claims her associates were "taken hostage" by Dushin and his people.
According to Klimina, it was Dushin himself who set fire to dry grass and other wood waste to clean up areas around his private lands.
"Each year, Dushin uses fire for cleaning space near his farming acreage and the fire then moves deeper inside the forest.... As far as we know, those people broke Mikhail Bars' jaw and collarbone, threw Yekaterina Anisimova into the river, and forcibly took the drone and mobile phones from them," Klimina said on April 29, adding that the gasoline in the canisters was fuel for the boat.
On April 30, the head of Klimina's administration, Marina Kosygina, told the TASS news agency that Bars had been hospitalized.
Dmitry Dmitriyev, a local lawyer, told RFE/RL that the fight most likely has political roots.
Klimina, who runs the district, belongs to the Liberal Democratic Party, while Dushin is a member of the ruling United Russia party. Klimina is up for reelection later this year.
More speculative (and largely obsolete now) answer from yesterday below:
We don't [yet] know what could have motivated these guys in particular, but concealing illegal logging has been mentioned in the past as a possible reason. However an more in-depth investigation by Rosleskhoz didn't find much evidence in support of this theory (although some cases of fires originating in illegal logging ares were noted). It's often enough the case that illegal logging is done with the tacit or corrupt approval of some local authorities as such logging is often done with documentation, just not with well justified one, e.g. declaring trees to be need to be removed for "sanitary" reasons, etc.
On the other hand, per an answer to a q of mine here, it seems that starting in 2019 controlled/prescribed burns are allowed in Russia in some limited circumstances (limited to grassland apparently). So it's not entirely impossible that those local officials may have been trying to do one of those and owing to the novelty of the regulations allowing it, they were beset upon by locals unfamiliar with that (law/regulations) change. It's curious however that no press story (of those linked) mentions this possibility. So either that was out of the question (in this incident) for some reason that's obvious to the Russian press, but not to me, or the journalists who wrote about it insofar are not terribly familiar with those changes either. Interestingly, the official announcement of the launch of an investigation does mention that the fire was in grassland (next to a forest). On the other hand, some of the press reports say that it was the mayor who caught his own vice-mayor (and his alleged accomplice). So that makes the simple theory that the locals were unaware of law changes (in re controlled burns) not so plausible. Alternatively, this might be some kind of score-settling between local officials.
Less nefariously, if I'm reading some the media reports correctly, the official roughed up by the locals may have been trying to shoot some kind of instructional video, perhaps on the dangers of forest fires and/or even how to do a controlled burn.