Forex market is a huge market with many players.
we can see ups and downs of forex due to many reasons, like economic indicators of different countries, gevenrment interventions and investors' behaviours.
however, the ultimate reason, and the main things to drive the market are mainly the following, i think
1. there are various countries in the word
2. different development model and different economic growth
3. global trade expedite the exhcange of the major currencies.
4. the problem with the fixed rate leads to the fluctuate currency rate.