Disclaimer: I'm no expert, this answer is based on my modest understanding of the situation at play.
The OPEC decision to cut oil production was meant to keep prices high: if every OPEC country were selling all the oil they could, it would drive prices down since there would be more supply than demand. So if every OPEC country agrees to sell only a limited amount of oil, the global supply matches the global demand and prices stay high. Naturally this works only if all the big oil producing countries play the game: it's tempting for a country to sell more than its agreed quota, especially if the price is high.
Russia is not an official member of OPEC, only an observer. As such, it is free to follow OPEC policies or not, depending on its own interests and strategy. I'm not aware of any official reason why Russia refused to follow this particular recent decision, but it looks like a rational economic decision: if many oil producers (competitors) reduce their supply and the prices are high, why would Russia limit their own supply if they have no obligation to?
"This refusal is cited as the cause of the 10% oil price crash on Friday alone, which was exacerbated by a further 30% crash on Monday."
This is not the full explanation: the Russian refusal led Saudi Arabia to cancel the original plan to reduce production (Guardian), and instead to flood the market with their own oil. This is what drove prices down sharply, especially in the current context of low demand due to the corona virus.
The Saudi surprise strategy could arguably be interpreted as a kind of tactical response against Russia's refusal to play according to OPEC rules: since Russia apparently wants to reap the benefits of high prices without sacrificing their own production, Saudi Arabia is telling them that "two can play this game". The effect is that all the oil producing countries are going to suffer from low prices, but some more than others:
The new strategy adopted by Riyadh appears to target Russia and US shale oil firms, many of which are known to have high production costs and lose money when crude prices fall below $50 a barrel for more than a few months.
Saudi Arabia can produce oil at a lower cost than Russia and US shale oil industry, so they are probably trying to use the opportunity of the Russian refusal to get rid of some competitors: Saudi Arabia can afford to take a temporary economic hit, whereas many shale oil companies may go bankrupt quickly. Strategically, their initiative is certainly also meant to assert their dominance on the oil market.
[Added: a similar analysis by Kevin Drum, found just after posting my answer]
The nickel version of this story is that oil prices started declining in February due to fears of lower demand caused by the coronavirus outbreak. OPEC tried to cut a deal with Russia to reduce output all around, but Russia balked. Saudi Arabia then decided to bring out its big guns, lowering prices immediately by about $7 per barrel and announcing that it would increase output in order to take share away from Russia. At that point the decline turned into a rout, with the price of WTI crude collapsing to $28 as I write this.