There was an email sent by Salesforce on March 1 with the title "Identity Confirmation Change with Spring ‘16 Release".
This goes over some changes, including a statement saying you'll expect more verification pages now.
The core message in that email was (emphasis mine):
At Salesforce, we understand that the confidentiality, integrity and availability of your data is vital to your business, and we take our obligation to protect it very seriously. In order to keep your data secure, Salesforce has changed the logic behind identity verification with the Spring '16 release. When users log in to Salesforce from a browser or device we don’t recognize, they may be prompted to verify identity, even if they log in from an IP address authenticated before the Spring ‘16 release. Users may be prompted to verify their identity more frequently than in the past, particularly those at organizations who automatically delete browser cookies for security purposes or do not specify trusted login IP ranges under Network Access.
There are a number of steps you can take to limit the amount of times the verification page is shown.
For more information, see Identity Verification Improvement with Spring '16 or Being Asked for an Identity Verification Code on Every Login