What can cause quick queries to run unusually slow?

Had an interesting query in my logs this morning:

DELETE FROM formobject WHERE objectid =
'C37EDD28-F46C-4DC4-A2D5FF042895C696';

It took *104 seconds* to complete. Here's the plan captured from the
log as well:

QUERY PLAN
-------------------------------------------------------------------
Delete (cost=0.00..6.33 rows=1 width=6)
-> Index Scan using formobject_pkey on formobject (cost=0.00..6.33
rows=1 width=6)
Index Cond: (objectid =
'C37EDD28-F46C-4DC4-A2D5FF042895C696'::bpchar)

We run this query all day long. What are things that might cause it to
take 1000 times longer than normal? It was not during our nightly
backup or any other intensive scheduled task that I'm aware of.
Autovacuum? Something else?

Brian