Why did God curse his own priest? Malachi 2:2?

God rebuked the Priests due to their failure to continue to teach the covenant that he had made with Levi. Levi showed loving fear of God and taught his ways and turned ones back from the wrong way.The priest's whose very position was to do the very same as Levi had done were polluting the alters with lame sacrifices and the made false praise from dead hearts.They stumbled many .Mal 1: 4-9

Because he violated his oaths to honor G-d in his rulings and he showed favoritism to certain parties in his position as judge, not just as priest.

The whole verse states it:

2 If you do not listen, and if you do not set your heart to honor my name," says the LORD Almighty, "I will send a curse upon you, and I will curse your blessings. Yes, I have already cursed them, because you have not set your heart to honor me.

See how it continues in verses 8 and 9:

8 But you have turned from the way and by your teaching have caused many to stumble; you have violated the covenant with Levi," says the LORD Almighty. 9 "So I have caused you to be despised and humiliated before all the people, because you have not followed my ways but have shown partiality in matters of the law."

if you read the whole chapter he is very clear that he is ticked off at them because how they have been acting and what they have been doing.