His motivation was the Massacre at York, as well as the anti-semitic riots that took place at his own coronation.
I think it would be interesting to see part of the actual (translated) decree (ellipses and italics are mine):
All the debts, pledges, mortgages, lands, houses, rents, and possessions of the Jews shall be registered. The Jew who shall conceal any of these shall forfeit to the King his body and the thing concealed, and likewise all his possessions and chattels, ..., and there shall be appointed two lawyers that are Christians and two lawyers that are Jews, and two legal registrars, and before them and the clerks of William of the Church of St. Mary's and William of Chimilli, shall their contracts be made.
And charters shall be made of their contracts by way of indenture. And one part of the indenture shall remain with the Jew, sealed with the seal of him, to whom the money is lent, and the other part shall remain in the common chest: wherein there shall be three locks and keys, whereof the two Christians shall keep one key, and the two Jews another, and the clerks of William of the Church of St. Mary and of William of Chimilli shall keep the third. And moreover, there shall be three seals to it, and those who keep the seals shall put the seals thereto.
... For every charter there shall be three pence paid, one moiety thereof by the Jews and the other moiety by him to whom the money is lent; whereof the two writers shall have two pence and the keeper of the roll the third.
And from henceforth no contract shall be made with, nor payment, made to, the Jews, nor any alteration made in the charters, except before the said persons or the greater part of them, if all of them cannot be present. And the aforesaid two Christians shall have one roll of the debts or receipts of the payments which from henceforth are to be made to the Jews, and the two Jews one and the keeper of the roll one.Moreover every Jew shall swear on his Roll, that all his debts and pledges and rents, and all his goods and his possessions, he shall cause to be enrolled, and that he shall conceal nothing as is aforesaid. And if he shall know that anyone shall conceal anything he shall secretly reveal it to the justices sent to them, and that they shall detect, and shew unto them all falsifiers or forgers of the charters and clippers of money, where or when they shall know them, and likewise all false charters.
There were two major benefits to this decree: one to the Crown, and one to the Jewish population. The Crown would have records of every transaction and could use them to tax the Jews involved. The Jewish moneylenders also benefitted, because any debtor wishing to accuse the moneylender of unfairness, or who tried to get out of repayment, now had to deal with a moneylender with the full weight of the Royal Exchequer behind him.
Documents from this office are extant for 1219-20, 1244, 1253, and 1266-87. (In 1290, all Jews were expelled from England by Edward I, or made to convert.)
One of these documents, in 1233, has an unusual feature: it is illustrated. The illustration above is part of it. The whole drawing is interesting as one of the earliest examples of Jews shown in a negative depiction. We're going to look at it very carefully next time.