Because C strings are arrays of characters terminated by a null ('\0') character, and arrays in C don't have bounds checking and don't come with a length attach

Because C strings are arrays of characters terminated by a null ('\0') character, and arrays in C don't have bounds checking and don't come with a length attached. If you allocate a 10-element character array, it can hold strings of up to 9 characters (plus the null terminator). But, the library functions ("runtime") don't know how big it is. So if you try to copy a 20-character string into that 10-element buffer, it will copy 21 characters. The first 10 will go into the array, and the next 11 characters will go... somewhere else. Depending on where in memory your array and other things are, something important will probably get overwritten with garbage.