It appears only oldest leaves are affected. We accept that deciduous leaves only last 1 summer but even evergreen leaves have a finite lifespan - usually 2-3 years. Then they become too old to work properly and have to die and drop off.
There are other possible causes of leaves turning yellow but What you are seeing is probably old leaves reaching end of life. The younger leaves above appear perfectly healthy.
BTW much bonsai advice is different depending on location. Please fill in location in your personal profile to get advice better tailored to your specific area.
Also, Just in case you don't know that azalea is an outdoors plant. They don't usually live very long indoors.