Storage devices have tendency of failure under some conditions, Like HDD failures are mostly because of its part failure, or extreme environmental conditions, magnetic field generation etc., or flash drives fails mainly due to electrical surges or electrical failures. If it has videos or any other data, it will be lost.
Human mistakes also can cause loss of data, e.g., accidental video deletion, or format of the drives.
Professional Video shooting instruments are heavy and tricky to handle. Inexperienced people can use them incorrectly causing videos or photos to be erased or lost. Also if these instruments encounter elements like, water or fire, that leaves damaged instruments and lost video/photo data.
System failure in which stored videos are present is also one of the reasons you can lose them. Reason for system failure can be power surges, sudden power cuts, OS failure, etc.
Many hotographers/Videographers use tools and software for editing their data. These software programs can be corrupted due to virus/malware attack, incompatibility with the OS, or lack of update installation and you can lose all your work completed through them.