Maximum profit
At the start of 1296 Floris V entered into an agreement with Philip the Fair, King of France. The Hollander promised to give Philip military assistance against the allies of England, but would not engage in active warfare against Edward I himself. In return for this service, Philip would pay him a life annuity of 1000 livres tournois per annum.
Should this agreement disrupt the marriage contract for his son, John (betrothed to Edward's daughter), Floris would receive compensation of 25,000 livres.
Floris had previously agreed to serve Edward against Philip, for a fee of £80,000 sterling. Prior to that, he had taken about £12,000 of Edward's money for his role in the Scottish succession crisis. This was separate to the £12,000 owed by Edward to Floris via the marriage contract.
It has been argued that Floris broke the English alliance because Edward moved the English wool staple from Dordrecht (in Holland) to Malines (in Flanders). However, there was no English staple in the Low Countries until 1313. A 'staple' was a fixed point, usually a transit port, for the compulsory sale of wool. Although Edward sent English wool merchants to Dordrecht in 1294, writs to custom officials show the merchants were free to trade anywhere in Holland, Zeeland or Brabant. Hence, this was not a staple.
The misconception arises from a statement made by English merchants during a trade dispute in 1320. They claimed that Henry III and Edward I had both set up wool staples on the continent. This was a lie – or, if we are being kind, an exaggeration. Henry III had arranged to sell English wool at Abbeville and St Omer in France, but not on a compulsory basis. Edward, as stated, arranged for wool to be exported at Dordrecht, but not as a staple.
That said, it is true that Edward shifted the centre of the English wool trade from Dordrecht to Malines. Whether it was a 'staple' or not hardly mattered, from Floris's point of view. He had lost money regardless. On the other hand, he had already taken over £100,000 in English sterling. To put that figure into context, the total cost of Edward's castle-building in Wales was £70-80,000.
The other point is that Gui of Flanders was Floris's enemy (and his father-in-law). Anything to Gui's advantage was, potentially, to Floris's disadvantage.
Overall, the most likely explanation (imo) is that Floris liked money, and had spotted an opportunity to cash in. Both England and France wanted him for an ally, so why not play off both for maximum profit? Great, if he could get away with it.
There was one snag. His son and heir, John, was in Edward's custody in London. If Floris did anything to anger the English king, John's neck was forfeit.
Oh, well.
