My response is that weakness of character makes people followers, eager to jump on "the bandwagon", whatever that happens to be. And that may simply be the old knee-jerk "safety in numbers" mentality. Weak people like to look around and see that they are surrounded by "friends" - - they're "safe", and liked, and with the "in" crowd.
Liberals are very easily manipulated and therefore prone to fall face-first for whatever propaganda the Democrat "mainstream" newsrooms happen to be pushing for their party this week.
Those are mostly symptoms. What you see as defective liberal character traits I see as universal failures that can apply to people of any ideology. Possibly liberalism attracts people of weaker character, but my research tells me that it is more that the ingrained paranoia of liberalism produces symptoms that can be classified as weak character.
The fact that liberals can become conservatives, with a noticable improvemnt in their attitude and behavior as a result of the switch seems to be evidence for my conclusion.
I invite you to read my essays if you truly wish to get my take on liberalism.