Everyone does worse on the 2nd half of the sections because you are suppose to do worse in the 2nd half of a Computer Adaptive Test... remember the questions get harder as you get the early ones right.. once you get to "your difficulty" level in theory you should be getting about half the questions right.. IE you would get one wrong, they reduce difficulty and you get next one right prompting harder difficulty question which you get wrong..
In practice this doesnt work exactly this way because your proficiency isn't the same across all topics.. To improve on the 2nd half of CAT's, you have to practice more difficult questions basically.
I have also been noticing that similar trend during my test preps. I have seen that some of my 2nd questions being wrong are as a result of rushing the final 10-15 questions, which is not good. It shows that I must significantly work on the way I mentally time myself.