In April and May
Your gross pension increased in April in line with the Consumer Price Index. This also means that the amount of tax deducted has also increased.
When you start receiving your State Pension
One reason your tax code change may change is when you start receiving your State Pension. The State Pension uses any tax-free Personal Allowance you have first and this may previously have been used for your Fund pension. This can mean you have less tax-free allowance being applied to your Fund pension and so pay more tax on it.