Saudi Arabia is keeping production high on purpose even though worldwide demand is weaker and worldwide supply is higher than before due to US fracking.
So while there are real reasons for the price reduction, Saudi Arabia is ensuring it keeps on going down by not cutting production which is what it normally would do.
Saudi Arabia, Qatar and the Emirates all have a lot of reserves so can afford lower income for years. Countries like Iran are weak economically, have been weakened further by sanctions and bad domestic economic policies, don't have reserves and have relied heavily on oil income.
The reason for Saudi Arabia's action is that they are weakening Iran's economy. And the reason they are doing that is they have ideological differences with Iran (Iran thinks Saudi's rulers are corrupt) and Iran has become the pre-eminent military Arabian power and also seeks to effect change through sponsoring those opposed to Saudi Arabia's rulers, especially since their ally Assad seems to have survived and the west is not being as tough on them on the nuclear weapons issue.