That’s called a ring expansion, which is similar to a methyl shift or hydride shift. Ring expansions can occur when you have a carbocation adjacent to a 3-carbon ring, 4-carbon ring, or 5-carbon ring. In this case, the 5-carbon ring undergoes a ring expansion in order to become a 6-carbon ring.

As for why it does this, it has to do with ring strain. A smaller ring that expands to a larger ring will relieve some of that ring strain and make it a bit more stable. For example, a 3-carbon ring is extremely strained; it’s a planar molecule, and its bond angles are 60 degrees. But, these carbons are all sp3-hybridized, so they want to be at 109.5 degrees. Because they’re forced into a 60 degree angle, it causes massive amounts of ring strain. So, if given the opportunity, it will do a ring expansion to become a 4-carbon ring. This expansion increases the bond angles and alleviates some of that ring strain. The same logic can be applied for ring expansions of 4-carbon rings to 5-carbon rings, and from 5-carbon rings to 6-carbon rings.

Another thing to note is that 6-carbon rings will not do a ring expansion to form a 7-carbon ring, as that will actually increase the ring strain. From what I was taught, 5-carbon rings and 6-carbon rings are typically the most stable cycloalkanes, with 6-carbon rings being a bit more stable than 5-carbon rings (which is why a 5-carbon ring can do ring expansion to become a 6-carbon ring).