What contributed to economic weakening of roman empire?

The heavy taxes helped to push the middle class into poverty.

There was what has been called the crisis of the 3rd century. One side of this was difficulties in containing Barbarian attacks and a string of usurper emperors who were hailed by the legions in their areas and tried to become rulers of their areas, leading to political fragmentation. On the economic side, these usurpers issued their own coins, thus increasing the money supply and creating inflation. Barbaric raids devastated the countryside on some areas, especially in Gallia. This created impoverishment which, together with inflation, undermined trade, which in turn affected artisanal production. The Romans never recovered from this. Poor understanding of economics was a key factor. Attempts devaluing the currency backfired. So did attempts to fix prices . They did not understand the the money supply was the real problem. Increases in the size of the army and bureaucracy increased the burden of public expenditure and (especially under Diocletian) undermined local administration. Over time, the urban economy in many towns collapsed and there was a large population shift to the countryside. This led to the beginning of a servile form of labour for the greatly increased farm labour force which was the precursor of serfdom in the Middle ages