anyone know why and how a website turn to adsense smart pricing ?

i have a site which perform very good on adsense for three month (daily profit 8-10 dollar) and out of sudden (this month) the site CPM drop ridiculous from 1.2-1.5 cpm to 0.3-0.5 daily cpm, CTR remain 1.3-1.8% .... daily income become 2-3 dollar ....

anyone face this type of problem ???