Huge increase of database partition

Hi the list,

Yesterday, i have had a big problem of disk space on my server, running
postgresql 9.0.13; with streaming replication.

Between 6pm and 7-30pm, i went from 45% used space to 99% (100% ?) used
space on my /data partition (who contains my postgresql/9.0/main/base ;
but does not contains the WAL; they are deported on an other partition).
- seen only after with munin -

Juste after that - still according to munin- , the partition suddently
decreases to its normal value (around 45%)
This peak represents around 165 Go of data ! ; beacause my partition
size is 315 Go.

I can't explain why, it's the first time i have something like this.
Usually, the size of the partition is very stable ; and grows very slowly.
Still with munin, i see that in this period, the blocks read per second
increases from around 20k to 80k, and i see that the disk utilisation
is around 100%, which confirms that postgres wrote a lot of data.

I don't understand how this can happen, have you ever seen things like
this ?

Thanks in advance,
Thomas