As with all mortgage rates, the banks set the rate at their discretion.
In the vast majority of cases, the mortgage contract specified what the tracker margin was. It was set at the time and seemed to be based on what other banks were charging. Usually they were lower for lower LTVs.
In most cases where people fixed the contract said "At the end of the fixed rate, you will be offered a tracker at ECB +x%"
permanent tsb had contracts which said "at the end of the fixed rate you will be on the then current tracker rate.". They then increased the tracker margins.
AIB said "at the end of the fixed rate, you will be offered an SVR, a fixed rate or a tracker at the then prevailing rates"
But they did not offer trackers as they claimed that they were not prevailing. They have now said that if they had offered them, they would have been prohibitive, which is why hundreds of customers are appealing this decision.
Brendan