siisch1 DISCONNECT requested - why?

[Dieter BSD]

FreeBSD 8.2 amd64
SiI3132 SATA controller
Seagate Barracuda LP 2 TB sata disk

Disk has been working fine for ~1.5 years, zero problems.
Suddenly:

siisch1: DISCONNECT requested
siisch1: siis_sata_connect() calling DELAY(100)
[ previous line repeated a gazillion times ]
siisch1: SATA connect timeout time=10000us status=00000000
siisch1: SIIS reset done: phy reset found no device
(ada5:siisch1:0:0:0): lost device
(ada5:siisch1:0:0:0): Synchronize cache failed
(ada5:siisch1:0:0:0): removing device entry
siisch1: CONNECT requested
siisch1: SIIS reset...

and the disk comes back. After umount/fsck/mount it is
working ok again.

Smartctl doesn't report any problems.
System has ECC memory, conditioned power, air conditioning, ...
I looked at siis.c but didn't find any clues.

What would cause a disconnect request out of the blue?