What does it mean if second half of 8-char string encrypted in 3DES is always identical?
DES has a block size of 8 bytes. Two blocks therefore come to 16 bytes.

It looks like Adbobe were encrypting passwords using two blocks of 3-DES in ECB mode. 

Because all these passwords are eight bytes long, the second block is empty and is just filled with zeros. The second block gets started at all because of the string-terminating NUL character at the position 9 of the password. The character that the rest of the block is padded with is also NUL. Because we're in ECB mode, this block encrypts to the same value each time. 

That's why all these passwords have the same eight trailing bytes: ioxG6CatHBw==. This is just the sequence of bytes you get when you encrypt 64 zero bits using their key.