The information I've gathered was that after Basil II,the empire came under the rule of a series of highly incompetent emperors and empresses.They pretty much depleted the treasury either because of luxurious spending,paying large pensions to bribe aristocrats or built a lot of vanity projects like churches to shore up their position.To further consolidate their position,they cancelled a good number of tax laws for the landowning elite,such as the one that forces them to pay the taxes of their poorer neighbors.All of this caused a budget imbalance.This caused a series of problems that included paying theme soldiers with debased coinage,which meant that theme soldiers would probably be poorer equipped and less motivated to fight.At this point,they also had a number regular army units which is also pretty expensive to pay.I didn't find out if they actually disbanded a number of regular army units to save money,but apparently,a solution was found by simply recruiting mercenaries on a short-term basis(I can't remember if this is accurate,but that's as far as I can remember).There's also the problem of occupying Armenia.The conquest of Armenia meant that the ERE effectively removed a strong buffer state between themselves and the Turks and this created a vacuum in the area around Armenia that they themselves are unable to fill(e.g. actually garrison the place) due to all the financial problems.The theme system fell because of pay difficulty and the fact that the army was much more offensively based,and was built around regular army units and mercenaries.The soldiers in the theme pretty much joined the army simply to get a piece of land and a nice supplementary pay.
As mentioned,these are only what I remember,the information I mentioned may not be accurate.