Most mortgage deals run for a fixed period, commonly two or five years, after which they usually move onto the lender's standard variable rate unless you arrange something new. This change can happen automatically and without much warning if you're not keeping an eye on the date.
The months before your deal ends are the right time to review your options. Your circumstances may have changed since you last arranged a mortgage, whether that's your income, your property's value or your plans for the next few years, all of which can affect what's available to you.
Reviewing early also means you can often secure a new deal ahead of time, ready to start as soon as your current one finishes, with no gap where you're paying more than you need to. Your home may be repossessed if you do not keep up repayments on your mortgage.