Last year I planted 5 holly bushes. Three of the five are doing fine (so far). Two of the hollys started dropping leaves last week and are nearly bare. What causes this and how can I reverse it?

Bulbcounter

hollys under 5 years old need plenty of moisture,water regularly but dont soak, and water well in fall before the ground freezes,
they also like acid soil, try putting a layer of organic stuff around em,like grassclippings,leaves,pine needles, coffee,tea grounds, fertilize with a with a slow acting fertilizer for acid loving trees. [hollys can be sensitive to over feritilizing]
aphids, scale, and leafminers can attack foiliage on stressed young plants,
hope some of this helps