Question: I must admit, I entirely did not understand why

> 
>  Queenie 
> 
> 
> 

chose to join Grindelwald at the climax of *The Crimes of Grindelwald.* She seems like the least likely, given her relationship with 

> 
>  Jacob.
> 
> 
> 

Can anyone explain?

Answer: 
When Queenie met with Grindelwald earlier in the movie, he swayed her by hinting that the current laws banning marriage between Muggles and Wizards/Witches would no longer exist and that if she were to follow him, that she would be free to marry who she wished.
This is just my opinion. Think about what time period we're talking about. It's the 1920s. If Queenie were perhaps pregnant, that would explain her despair on the streets of Paris, her frustration at Jacob refusing to marry her and her feeling like she didn't matter. Women in pre 1970s were treated horribly if they were pregnant without being married and Queenie finds herself wanting to marry a man she would be imprisoned for dating, let alone having a child with. She cannot go back to America and Jacob won't marry her for fear she'll get in trouble. That makes her motivation more understandable at least to me.