Hostas begin to push their new
growth up from the ground in the form of "bullets" which
are actually folded leaves that are held tightly
together. As the weather warms, the leaves unfurl and
open into their full width and length.
If the bullet is exposed to
sub-freezing temperatures, certain cells on the outside
of the leaf tissue will freeze. If they freeze enough to
cause the rigid cell walls to burst, the tissue is dead.
Later when the leaf fully expands, the broken cells
cause a split in the tissue. You can tell a frost crack
in that the jagged edges of the opening will fit back
together like a jiggsaw puzzle.
So, if certain hostas have emerged
and the temperatures are going down into the 20s, find
some way to cover them. Individual clumps could be
covered with a plastic pot or tarp. Once the leaves are
damaged, of course, they will never "heal" back
together. If they bother you, cut them off and wait for
new ones to emerge.