"Improper storage" of master tapes refers to (at least) two different things:
1- The temperature and humidity of the room the tape is stored in. Major tape vaults (Capitol Records in Los Angeles for one) are climate controlled, being not too hot or cold, and with humidity not too high or low.
2- The tape is stored after being fast-forwarded or fast-rewinded, rather than at playing speed. The high speed forwarding and rewinding brings each layer of the tape into closer proximity to the next (because the tape is wound tighter), causing the magnetic signal to "bleed" from one layer to the next. If you hear a recording with pre or post "ghost-echoes" (I have), that is probably the cause. A tape that is in it’s middle at the end of a session is allowed to run at normal running speed (15 or 30 ips) to it’s end, the tape then being "tail out", and stored that way. That is why tapes are usually stored tail out, not for any other reason. The next time the tape is used for new recording it is rewound back to the beginning of it’s unrecorded portion.