ATET, could it be made "more concurrent" ?

PostgreSQL 9.1.x

I try to enable my trigger
  "ALTER TABLE cool_tbl ENABLE TRIGGER trg_for_cool_tbl;"

I notice that it gets blocked by another backend doing
  "select xxx from cool_tbl;"
(of course, my pain is if this transaction lasts for minutes, instead of se=
conds)

I understand that [postgresql.org/docs/9.1/static/explicit-locking.html] te=
lls us
The request for "AccessExclusiveLock" on relation R will be blocked by the =
existing "AccessShareLock" on relation R.

But I cannot reconcile that with . . .

How could doing ENABLE of a trigger hurt the transaction that has thus far =
only done a SELECT on the table ?

Thanks for your insight,
-dvs-