So Chimping is a term used in Bangalore quizzing circles (or maybe Mumbai or Pune, doesn’t matter). Quizzing folklore goes that a guy asked a question like “Which species of primate can <something something>”, and the question held no clues about the answer. So essentially each team had just just guess one or the other species of ape/chimp/monkey, and hope it was the one. This is a bad quiz question because you have no way to deduce the right choice from a small set of equally probable answers.
Something similar happens in thrillers and detective novels that don’t develop their characters. As a reader you’re invited to guess the outcome/villain in these books. Typically there’s a small set of possible outcomes: the bad guy gets caught, bad guy gets away, the hero lives, the hero dies, the heroine/supporting character is the bad guy/girl… For detective novels the outcomes are pretty much, “the bad guy is A/B/C/D”, because you need to have a limited number of characters, all suspicious.
Now if you don’t develop the characters to make them feel like real people - to make the reader develop an understanding for this or that person, to root for the detective or the wronged female, then all that’s left is a set of plot points that unfold one after the other. Because there are no hints to tilt the reader towards or away from some outcome, she’s just going to “chimp” - guess at all the possible outcomes one by one. Whichever one happens, there's no "aha", or sense of deduction from the character-clues that came earlier.
As a writer you need to do one of two things: either set up expectations for the characters and then fulfil them in unexpected ways (so Lee Child basically), or else set up expectations and destroy them in an unexpected way (so basically, Tamil tearjerker movies where the hero dies pointlessly). Either of these requires the reader to empathize with the characters first. If your reader is able to label the characters as “The Hero”, “The Love Interest”, “The Bad Guy” and not really care about them, you’ve failed. Don’t let the reader be reduced to Chimping.
PS. This is where the movie Se7en played with the genre and really managed to shock viewers.