Definition of implication: why does p(x) v q(x) become ~q(x) -> p(x) rather than ~p(x) -> q(x)?

The easiest way to prove equivalence is to draw out a truth table and then compare the values. I'm going to show a truth table using proposition logic, it's the same result as using predicate logic. 
P(x) v Q(x)

P |Q || PvQ  || ~Q->P  <Notice how this column matches the PvQ but if you were to 
---|---||||  <continue the truth table with ~P->Q it would not be equivalent
T   T      T           T   
T   F      T           T
F   T      T           T
F   F      F           F

Let me know if you would like an example, if the truth table doesn't help. 

Hope i can you!