 Why was Humphrey being pick-pocketed so much?
The pockets of Mr. Humphrey Fownes were being picked outrageously. What made it possible was Humphrey Fownes' abstraction; he was an uncommonly preoccupied individual. In the ensuing contretemps they emptied his rear pants pockets, got his wristwatch and restored the contents of the handkerchief pocket. It all went off very smoothly, like a game of put and take—the sole difference being that Humphrey Fownes had no idea he was playing. Humphrey Fownes strode through the puffs of falling glass still intrigued by a temperature that was always 59 degrees, by a humidity that was always 47%, by weather that was always Optimum. It was this rather than skill that enabled the police to maintain such a tight surveillance on him, a surveillance that went to the extent of getting his fingerprints off the postman's bag, and which photographed, X-rayed and chemically analyzed the contents of his pockets before returning them. He came to see that genuine quirks were rare and, as time went on, due partly to his own small efforts, rarer. Lanfierre was almost proud of Humphrey Fownes. You've got winds in your brain, Lanfierre, breezes of fatigue, zephyrs of irrationality—" At that moment, all at once, every last window in the house slammed shut. Fownes stopped on the porch to brush the plaster of paris off his shoes. Or, as Fownes hoped it might appear, the illusion of the domed city vanished and was replaced by a more satisfactory, and, for his specific purpose, more functional, illusion.
 for Lanfierre to get to know Humphrey's personality better
 to plant information necessary to arrest him  it's a typical behavior in this city  people typically pick-pocket him because he's distracted  for Lanfierre to get to know Humphrey's personality better