The vile and hideously named Dursley family were known for their wicked temperament and horrible treatment of young orphan Alan Rickman, Jason Isaacs, Gary Oldman and Maggie Smith, among others.
Following the vicious murder of both of his parents, young Harry was sent to the Dursley's home. The Dursley matriarch, Petunia (Fiona Shaw), was the oldest sister of his mother Lily. Whilst there, Harry was brandished as the bad sheep of the family, forced to sleep underneath the staircase, and kept ignorant of his magical background until the magic literally burst at the seams.
Though Harry's cousin Dudley (Harry Melling) eventually saw the errors of his tortuous ways – that is, once Harry saved him from a cruel butchering by a Dementor – there may have been an alternative reason as to why the Dursleys loathed their nephew so much. As Harry ventures through his schooling at Hogwarts and engages in his outstanding battle against Voldemort (Voldemort killed Harry's parents, his already wretched soul had fractured, and one of the remnants had latched itself onto the infant boy, turning him into a horcrux. Given the fiendish and evil nature that was lurking inside of him, perhaps the mogul Dursleys had a toxic reaction to Voldemort's presence inside of Harry, cursing them into hating their orphaned nephew.
The Sirius Black in The Order of the Phoenix, was possessed by Voldemort, who fed off of his shock and anger.
With that in mind, given that the Dursleys' familial situation has been greatly stirred up – not only has one of its been murdered by the most horrendous being in the world, but now they have the "burden" of raising a new child with similar, and potentially dangerous abilities – it is hardly unreasonable to suggest that Harry's horcrux could have affected the muggles. If these cursed soul fragments have the ability to tramp the will power of moral-minded wizards, what's to stop them from tampering with the minds of a band of snobbish, pig-headed and comparatively weak human beings?
