How does mattie react when she sees Eliza? Why do you think Mattie can finally “cry like a river” and let out her emotions

Mattie cries but is happy to see her. Mattie says, "I feel like a turncoat, a traitor." A turncoat is a historical reference to someone who switches political sides, like Benedict Arnold in the American Revolution.  Mattie started crying because she feels safe.

Explanation:

1.the hosts on two opposing mountains stood,  

2.thick as the foliage of the waving wood;  

3.between them an extensive valley lay,  

4.o'er which the gleaming armour pour'd the day,  

5.when from the camp of the philistine foes,

6. dreadful to view, a mighty warrior rose; in the dire deeds of bleeding battle skill'd,