When lightning strikes, the phone network and power network are often driven to different electric potentials, meaning a current will flow between them. When that happens equipment that is connected to both the phone line and the power will act as an (expensive) fuse.
With only one connection (power or phone) the whole device tends swing at the same potential. Damage can occur if it arcs to ground, but that's less likely as in a device the isolation to ground is typically higher than between the power and data parts of a circuit. (Extra bad news if the arc to ground happens via a person.)
Incidentally, if a lightning strike is nearby a typical consumer surge protector will improve the odds but probably won't prevent damage.
With only one connection (power or phone) the whole device tends swing at the same potential. Damage can occur if it arcs to ground, but that's less likely as in a device the isolation to ground is typically higher than between the power and data parts of a circuit. (Extra bad news if the arc to ground happens via a person.)
Incidentally, if a lightning strike is nearby a typical consumer surge protector will improve the odds but probably won't prevent damage.