I think the basic reason that this kind of thing happened to Odo, especially early on, was that they wanted him to be just as vulnerable as the other characters, rather than unstoppable and basically invulnerable like the liquid metal Terminator had been.
The writers understood that fans would tend to theorize that Odo should be very difficult to harm (as of course they still do) and didn't want to take his character in that direction.
That's especially important when you consider that the whole concept for the Founders later became that they had been persecuted for centuries by solids until they ended up forming the Dominion in response to that. How could these potentially invulnerable killing machines have been persecuted and hunted by humanoids without advanced technology?
Later in the show the writers got a little more comfortable with showing the changelings doing some impressive things, but the original concept really was just shape-shifting, not superpowers.
For something resembling an in-universe explanation, I theorize that, when a changeling is in his natural state, he is extremely sensitive and vulnerable everywhere, a bit like having exposed nerve endings everywhere or having your consciousness diffused throughout your body.
So I might be able to "knock out" an unsuspecting changeling by striking him anywhere and generating a pain stimulus that is so strong the changeling passes out. However, when the changeling is able to anticipate the blow, he could harden specific areas of his body and allow his consciousness to flow into other areas, and therefore not be affected by my attack.
When Odo is in humanoid form, the situation might be somewhere in between. While mimicking humanoids, his consciousness seems to become focused in his head (for example he sees with his eyes, not with his hands), so maybe his head becomes vulnerable much like a humanoid's, thus allowing him to be knocked out in much the same way.
