Fracture. Usually embrittlement in the weld zone. There are a couple of ways it can be introduced/ manifest, but the cyclic nature of the loading will result in growth of a flaw. Once the geometry of the connection is altered, the entire joint mechanics can get thrown off.
Note- I’m a structural engineer, but not an expert in fracture mechanics or weld metallurgy. I’ve seen failures like this but I know just enough to know when it’s beyond my competency. The details here are something I lean on other specialists for. This is definitely a juicy one.