reasons

• Tostig taxed Northumbria too heavily - Danelaw areas were not used to heavy taxation, so Northumbrians resented him for it
• Tostig had ordered the assassination of high born Northumbrian rivals whilst they were his guests
• Tostig was a southerner Northumbria had always been governed by northerners - the south AS king had generally left the North to govern itself
• Tostig's friendship with Malcolm III, King of Scotland, meant he didn't defend Northumbria from Scotland attacks. Whilst Tostig was away in 1061, Malcolm invaded and caused a lot of destruction, Tostig didn't retaliate, he only made peace with Malcolm
• Tostig had been unjust - imposing laws (often from the south) and abusing his power to get rid of his rivals. Tostig's enemies complained that he falsely accused people of crimes in order to take their money and land from them