Supporters of the first President Bush were upset because he raised taxes after having explicitly promised not to do so. His promise had helped to convince many conservatives that he was a worthy heir to Ronald Reagan.
Bush was not, at first, seen as a true conservative. He had famously derided Reagan’s economic plan as “voodoo economics” when they were both running in the Republic primaries in 1980. Because he was not seen as a true conservative, Bush had to shore up his conservative credentials when he was running for president in 1988. He did so, in party, through his “read my lips, no new taxes” mantra. He also promised not to raise existing taxes.
Once in office, Bush had to deal with a Democratic Congress. Congress would not come to a budget agreement with him until he agreed to raise taxes. When he did so, his supporters felt that he had gone back on his promises and that he had violated an important conservative tenet. Conservatives strongly believe in lower taxes and felt that a true conservative would not have agreed to the tax increase that Bush agreed to.