Okay, looks like everyone's up in the air on why she chained him to the mast in the first place. Some say it was because she was afraid, others have different reasons.
I agree that having a capable, usually level-headed girl like Liz suddenly lose it to the point of leaving someone to their death is a little extreme. We saw how well she kept her head during the raid of her house, her dealings with Barbossa, battling on the ship, burning the rum, fighting off Davy's crew. She's strong, physically and mentally.
She also seems very, VERY happy to have Jack back. True, it can be a scary thing when you learn someone loves you so much they would come face death for you. I do believe he came back mostly for her, but also that her words got to him and he is a good person deep down. But she was thrilled to see him and cuddled his leg (I still wish she hadn't done that), so up to the point where she says they have to get to land, she seems happy to have him there.
Which is why I don't understand how she could go a whole 180 in such a short time. Right after the Kracken, none of them really have a lot of time to slow down and think, including Liz. She doesn't have time to think "Jack came back for me. He's in love with me and I'm supposed to be marrying Will." I'm sure the main thought on everyone's mind was SURVIVAL.
So Liz gets it in her head that the Kracken is after Jack not the ship. Jack wasn't on the ship when the Kracken attacked, so her logic may be a little off, but couldn't the Kracken's be off too? As long as Jack remained alive and with them, he was a danger to them. I really think the main reason Liz did what she did was to save herself and the others.
But that doesn't explain the kiss, and here is where that comes in: Liz is falling for Jack, hard and fast. Her noble idea forming, she decides to kill two birds with one stone and kiss him, able to make the excuse it was to distract him. And we've all seen the clip a million times, it's hot and they're both really into it and she succeeds in seducing Jack.
And she leans in for another and he's all too happy to oblige her, and THAT'S when I think she started getting scared just how much he loved her.