Bead and track
Liner bead popped out
Swimming pool liners do not typically pop out without cause. There are two main reasons why your liner may have slipped the track.
First, your liner may fit poorly or be incorrectly sized. Upon initial installation, these liners are often coaxed up into the track with patience and a lot of extra pulling and tugging. As vinyl liners age, they lose some of their elasticity. Combine this with slight contraction in cold weather and the floating effect of a rising groundwater table during winter rains, and a poorly fitting liner can easily pop out of the track.
Second, an old liner track may be stretched out and may no longer hold the bead as tightly as it once did. In some pools, both problems are happening at the same time.