There was a recession going on at the time. Unemployment was fairly high leading people to seek a new form of leadership, or look for some kind of change. It has nothing to do with party or politics. The general rule is that whoever is in power, they often take the blame if the economy goes bad. Of course, there are a lot of complicated factors at play. No one person controls the economy.
President Bush was seeking re-election going against some members of his own party. Those members thought that the current president wasn't doing enough of their agenda. So in a sense, the party split in a way between the Bush group and the other side of the party. This split helped elect Clinton. What also helped Clinton was his personality and ability to speak to people on a personal level. He was able to connect better with the average voter. In contrast, Bush seemed distant and a bit elite (in terms of wealth) to be able to understand the hardships of the recession. Many people point to the debate in which Bush checked his watch which seemed to signal doom for his campaign. Why is that simple move bad optics? Because it tells people that he doesn't seem too interested and would rather be somewhere else. It's possible that he didn't mean to put out that signal but that's how the public perceived it. That is one example that helped put a nail in the coffin so to speak.