I'm not overly convinced there's any covert ops, or even domestic sabotage going on. Russia is infamous for its industrial fires and accidents. Remember the countless ammo depot explosions over the last few years? Remember last year or something, when there was some nuclear ship having trouble with its reactor? And some people I think died to avert a massive nuclear catastrophe? This shit happens all the time in Russia – most of which we Westerners never even hear about.

With sanctions (& a terrible, corrupt economy generally) hurting maintenance efforts, by preventing import of vital parts & materials, not to mention cuts to pay & all the incompetence, laziness & 'shortcutting' that'll bring, these all could just simply be the wear & tear starting to really hurt.

The simplest explanation IMO Is simply that the Russian Military was involved in firefighting services and they're not there anymore so fires that would be a moderate disaster in peacetime are allowed to turn into infernos.