Probably an eyewall replacement cycle. Here's what Weather Underground had to say about it:
In the early afternoon, Matthew began forming concentric eyewalls--typically the prelude to an eyewall replacement cycle (ERC), where the inner eyewall collapses and a new larger-diameter outer eyewall takes over. At 3 pm EDT, the Hurricane Hunters found
two concentric closed eyewalls--an inner eyewall 12 miles in diameter, and an outer eyewall 75 miles in diameter. Just three hours later, the inner eyewall had shrunk to 8 miles in diameter, and the outer one to 57 miles in diameter. At this time, a portion of the inner eyewall passed over
Settlement Point on Grand Bahama Island, bringing sustained winds of 75 mph, gusting to 86 mph (and as high as 105 mph at 6:11 pm EDT). Around 7 pm EDT, Miami radar indicated that the inner eyewall had reconnected to the outer eyewall via a spiral band, while the outer eyewall continued to contract.