Simply stoooopid!!!
I believe Mr. Trump feels that Bush elevated the threat of WMD to a level that it shouldn't have been (i.e. nukes). There is no doubt that Hussein had chemical weapons at one time and he probably possessed bioweps as well. Still, there were some in the intelligence community and some inspectors (Scott Ritter for one) who were arguing that there were no WMDs in Iraq.
My disatisfaction with Bush isn't over the invasion of Iraq, but over the execution of the war. It is evident (IMO anyway) that he wanted to keep his tax cuts in tact by allowing Rumsfeld, and directing Casey, military et al to fight a limited war (war on the cheap) instead of employing the proven military tenent of overwhelming force. Limited war resulted in the foreign Al Qeida terror groups being allowed to pour in over the unsecured border and sparked the insurgency after the fall of Baghdad. Together with the looting, crime and widespread anarchy it will serve as Bush's lasting legacy of failure as Commander in Chief.