Cable "burn thru" is one possible cause of the failure. Most long US undersea fiber cables are DC powered using the ocean as a return path for the DC power.
If the cable sheath is damaged the cable, in essence, shorts to the ocean. which can cause the copper conductor in the cable to burn away or dissolve - like electroplating in reverse, for lack of a better description.
They fish the cable up, splice on a new section of cable and run along till they find the next 'whole' section, another splice, and testing to confirm the repair.
Damage can be caused by fishing gear - the cable are normally found on charts - or by anchors, seismic events or chafing from tidal or current moving the cable across the seabed. More can be found
here with links to photos.