UNIX 4.2 thrashing - the cause?

[Hur, Jinho]

Somewhile ago, our VAX 780 showed a disastrous performance.
It couldn't even echo the keystrokes from the terminal not to say of
any works. 'uptime' reported 14 users, load average about 4.5.
I only had to halt the cpu (^P) followed by a reboot.
As far as I can guess, it surely is a thrashing, a rarely occurence.
If you have any experience, please send me relevent information.
Like the cause you found, the remedy, etc.
Here is the disk usage for reference ( any hint? ).

Filesystem    kbytes    used   avail capacity  Mounted on
/dev/hp0a       7421    6519     159    98%    /
/dev/hp1h     137616  120835    3019    98%    /usr
/dev/hp1g      74691   65354    1868    97%    /usr/spool
/dev/hp2h     137616  120775    3079    98%    /va <- user area
/dev/hp0g      38639   34747      28   100%    /vb <- user area
/dev/hp2a       7429      30    6656     0%    /tmp
/dev/hp2g      74691   62096    5126    92%    /UDS <- utilities
-- 
real:	Hur, Jinho	Dept of Computer Science, KAIST
uucp:	..!hplabs!kaist!jhhur
csnet:	jhhur%kaist.csnet