How close is it to that window? Sometimes light coming in through the glass can work like a magnifying glass and burn leaves. If the rest of the plant is healthy it may be getting a sunburn from direct light. Youay be noticing this condition now because the sun is at a different point in the sky during the summer.

If the spots are black is sunburn if they are brown it may be a fungal disease or fungus.Take a look of google images and see if they match your problem just type in yucca fungal