the watcher in the water

[akhtene]

But why grab Frodo (A small hobbit that was getting lean)?

Could it be that the Watcher just grabbed the last of the group? In "A journey in the dark" it's said that "the others swung round" hearing Frodo's yell, so he was definitely lagging behind.

Another question is bugging me: What for did the Watcher slam the door? Was it intentional or the groping tentacles just pulled at whatever they seized? Any ideas?