JWST solar panels have limits on what angular rate of rotation they can be deployed at. After separation, there is rotation introduced into the spacecraft, which it has to dampen with thrusters until the angular rate is within acceptable limits. In this case the tip off (rotation after separation) from the Ariane 5 rocket was basically perfect (compared to the conservative estimate) which meant the spacecraft had to spend less time burning RCS to get within limits and thus solar panel deployment was sooner.
This is confirmed on the JWST blog:
That [solar array] deployment was executed automatically after separation from the Ariane 5 based on a stored command to deploy either when Webb reached a certain attitude toward the Sun ideal for capturing sunlight to power the observatory – or automatically at 33 minutes after launch. Because Webb was already in the correct attitude after separation from the Ariane 5 second stage, the solar array was able to deploy about a minute and a half after separation, approximately 29 minutes after launch.
