Must have been an issue with the breaker itself. Once a replacement was installed everything was fine again. I guess it was just going bad or just was more sensitive from age. I often wonder is better to have a GFCI breaker or a standard breaker with individual GFCI outlets 
The best compromise is something that's called an 'Alamat' here - a combined GFCI and conventional circuit breaker in 1 device. That means that every circuit is ground fault protected, and you can use conventional outlets.
But we still often use 1 GFCI per 3 circuits, simply for cost reasons.
It all depends on local electrical code as well as your personal wishes. The more GFCIs you use, the lower the trip current can be, but the higher the cost.
I'm not too enthousiastic about only having a gfci inside each outlet, because that does not protect the wire runs themselves. In other words, if you drill into a power line, you can get zapped without the GFCI ever knowing about it. Same with moisture intrusion into junction boxes. Power won't shut off even if the wires are practically submerged, but if they dry out and damage is done, you can have arcing with the associated fire risks.
An AFCI can protect against that, but ideally you catch the problem before arcing starts.