The smartctl data you show up there looks perfect; the disk seems healthy. The reallocated sector and pending sector counts and errors are all zero.
Now, the fact that the disk itself (spinning platter and moving heads) are healthy doesn't always mean that the disk interface it also healthy. But there is no indication of a bad interface in your case. In theory, one could have a bad SATA cable, and given the state of hardware drivers, it can cause FreeBSD on commodity hardware to come to a sudden stop (no writing of crash dumps, no reboot, machine completely frozen). Been there, done that, had to pull all disk cables out one at a time until it started working again. But that's not your problem: (a) You are getting a crash dump, it it's telling you what the reason is, which is not hardware related. (b) The machine reboots fine.
As already discussed by others above: The reason is a file system inconsistency, meaning a data structure on disk has become garbage, in a directory. It's the kind of inconsistency which causes the kernel to deliberately crash. This could theoretically be caused by the disk hardware (undetected read error, causing corruption in a data structure). But more likely is a bug in the FreeBSD software. There is no way a normal user-mode application (like transmission-qt) can legally cause this. The only way to cause this without assuming an OS bug would be if root deliberately overwrites a disk, which is highly unlikely (unless someone has been playing with dd command, which hopefully doesn't happen).
What should you do? Save your data, and then reformat the problem file system. If you have a spare disk drive, here's what I would do: Get the spare drive, format it with the desired file system, copy the surviving data onto the spare (hope the copy process survives without a crash), put the spare into the running system, take the problem disk out, and reformat it with the file system, and you have another spare again.
Now, the fact that the disk itself (spinning platter and moving heads) are healthy doesn't always mean that the disk interface it also healthy. But there is no indication of a bad interface in your case. In theory, one could have a bad SATA cable, and given the state of hardware drivers, it can cause FreeBSD on commodity hardware to come to a sudden stop (no writing of crash dumps, no reboot, machine completely frozen). Been there, done that, had to pull all disk cables out one at a time until it started working again. But that's not your problem: (a) You are getting a crash dump, it it's telling you what the reason is, which is not hardware related. (b) The machine reboots fine.
As already discussed by others above: The reason is a file system inconsistency, meaning a data structure on disk has become garbage, in a directory. It's the kind of inconsistency which causes the kernel to deliberately crash. This could theoretically be caused by the disk hardware (undetected read error, causing corruption in a data structure). But more likely is a bug in the FreeBSD software. There is no way a normal user-mode application (like transmission-qt) can legally cause this. The only way to cause this without assuming an OS bug would be if root deliberately overwrites a disk, which is highly unlikely (unless someone has been playing with dd command, which hopefully doesn't happen).
What should you do? Save your data, and then reformat the problem file system. If you have a spare disk drive, here's what I would do: Get the spare drive, format it with the desired file system, copy the surviving data onto the spare (hope the copy process survives without a crash), put the spare into the running system, take the problem disk out, and reformat it with the file system, and you have another spare again.