Why might my knee pop & give out occasionally?

Giving way from pain. Pain generated reflexes can cause a sensation of instability and giving way, much like a frank ligament tear (acl) whereby the injured structure (meniscal tear, cartilage lesion, loose body or patella subluxation most commonly) cause sudden catching/popping and pain that triggers aberrant muscle firing beyond one's normal control. This can result in your knee buckling and giving way.
Depends. If you have a prior history of injury, the popping may be a sign of instability such as patellar or knee joint (such as the anterior cruciate ligament). If you do not have a history of injury, this may be related to patellofemoral maltracking (how your knee cap moves along your femur). Therapy often helps alleviate symptoms and helps to build strength (core, buttock, leg).