This is from HAL's fstab-sync. This functionality is primarily for removable media.
The paths in /media/ are part of the Linux filesystem hierarchy standards. Slackware is perhaps the only distribution today that neglects to use this.
http://www.pathname.com/fhs/pub/fhs-...EDIAMOUNTPOINT
As there are some users that still love being back in the UNIX stoneage and manually mounting CDROMs and USB drives, you can discontinue the use of the HAL automounter by disabling the rc.hal stuff in /etc/rc.d/ . However, these features are desired by the majority of Dropline GNOME users, and they are enabled by default. You will find that all of the Slackware GNOME desktops (Dropline/FRG/Gware) make use of HAL, which interfaces with the 2.6 kernels (HAL probably kicked on after you updated your kernel, as the script checks for it on startup). It's hard to tell if Slackware will ever officially adopt HAL, but since KDE and other desktops are also making use of it, it's very likely to appear in a future release if the userbase demands it.
Alternately, if you must insist on using /mnt/ , you may modify the policy to use it instead of /media/ .
/usr/share/hal/fdi/policy/10osvendor/10-storage-policy.fdi
You can copy that file to a subdirectory of /usr/share/hal/fdi/policy/ to create your own policy. Modify it to use /mnt/ instead of /media/, and save it.
If you need more assistance, you should be able to search google or the Dropline forums on how to properly edit this file to make it automount to the preferred location.