My theory is that the Orcs, hearing that a force of Dwarves had occupied part of their old halls at Khazad-dûm, decided to exterminate them all; both to avenge Azog and his bloody and humiliating defeat at the Battle of Azanulbizar and to discourage any further attempts to re-colonise the Mines.
If this was indeed their goal, damming the Sirannon would be the best action to take, cutting off as it did the Dwarves' most logical direction of retreat from a Westward attack "up from east up the Silverlode".
My version of events is that the Orcs didn't attack from two directions at all. They sent a small force to dam the Sirannon, under strict orders not to arouse any suspicion, which would explain why they left the trees alone. Once this was done, Balin was assassinated either as a premeditated opening move or, which I think more likely, a deft piece of opportunism by one Orc. Then the main assault began.
Why a single-pronged attack? My opinion is that the Orcs had formed an unholy alliance with some primordial creature, which they set to watch the Gates, preventing any escape in that direction. They did this as quietly as possible so that the Dwarves would fall back before the larger force, thinking their point of egress clear. Thus by the time the Dwarves realised that they were trapped it was too late to do anything but hole up in the Chamber of Mazarbul, the Orcs having carried the Bridge by sheer weight of numbers.
Now, in my view the only reason why the Orcs should have employed this strategy, which must have been terribly costly in terms of lives, is if their only intention was destruction. They knew that there would be a last stand, and they knew that it would entail heavy losses on their part, but Orcs aren't ones to care about the numbers of their dead, presumably taking their lead from Sauron. Since any Dwarven losses at the Bridge would mean weaker resistance at the end they decided to chance it, relying on their numeric advantage. This advantage carried them to victory, as we have seen.
I agree with Kuruharan about the use of the word "Watcher" in the Book of Mazarbul; it does seem to imply that at the very least the defenders interpreted the creature's role as that of sentry, preventing their escape. It could even be that the Orcs were aware of something nasty in the Mines that the Dwarves had yet to discover, and decided to free it to aid them in accomplishing their objectives. I recall no mention of any Orcs being taken by the thing in the water.
In my view, therefore, the last lines in the Book mark the culmination of a cunning piece of crowd-control and extermination:
Quote:
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...the pool is up to the wall at Westgate. The Watcher in the Water took Óin. We cannot get out. The end comes.
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I think that the drums in the deep were a taunt; a final psychological blow to demonstrate that this was the plan all along; that now the end had finally arrived.