In the framework of the 2nd European Payment Services Directive (Payment Services Directive 2 - PSD2) which requires enhanced security measures for electronic transactions, you may be asked to enter the card's PIN for payments under the country's contactless limit (for example 100£ in the UK).

Specifically, in contactless transactions, that exceed the limit of 100£ you will be required to insert your card in the chip reader and enter your PIN.