Why do the leaves of a baby's breath bonsai curl, turn yellow, dry up, and drop?

Baby's breath (Gypsophila paniculata) is a perennial herbaceous plant that blooms from June to August, and is generally grown as a cut flower material or in pots. But many enthusiasts also train it into a bonsai for viewing, and it is very beautiful, especially in the flowering season, when the gorgeous large flower balls are even more delightful to the eye.