Why did Cassidy decide to testify?  

After all, she was a top Aide to Mulvaney, who was trump's chief of staff for quite some time.

Her original attorney was someone with deep maga roots, but she decided to part ways and found a more down to earth attorney that probably told her she should tell the truth 

Old guy: Let's say you are right ... she's lying ... For someone who worked in a republican administration, it is and would be political suicide to testify in the way that she is ...

So, what would compel her to lie about all these things, Old guy?

Hint: She has nothing to gain and everythign to lose by testifying... There's only downside in it - 

Old guy, you have zero logic in this - You have to look at motivation as to why peope do these things...then you'll figure out that it's more than likely true, everything she's saying