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 seconds)

I understand that [postgresql.org/docs/9.1/static/explicit-locking.html] tells 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-