You should start by determining whether this is tripping from:
- Ground fault
- Arcing fault
The breaker will tell you - google up the instructions for the breaker and it will explain the procedure (if any). Some models need electricity to store the fault codes, so pulling the breaker out of the panel and swapping may have destroyed that information.
The ground fault case has been discussed at length by manassehkatz.
An overload is either the circuit being generally overloaded, or an intermittent short.
An arcing fault is current "leaping" across a poor electrical contact, due to a defective or badly installed splice, or faulty device. It is making tremendous heat at the faulty connection, and this can cause burn-up of the wires, box and even start a fire. It was invented to reduce electric blanket fires, but was found to be very good at detecting backstabs (whoops, said the quiet part out loud) arcing at bad connections in walls. That is why it is broadly required now.
If you've ever heard someone hooking up stereo speakers with the amplifier turned on, and there's a "crinkle crunch" sound... Or a bad microphone cable can cause this as well. Or plugging in headphones. A digital signal processor inside the breaker is "listening" for this "sound". This is not a perfect science, but still, the first thing I would check is wire connections along the route.
I would focus on opening up every junction box in the circuit, and closely inspecting any connections that might be faulty backstabs. Seriously, the problem with backstabs is they have a long track record of connection failurs, and are not inspectable, except by wresting the wire out of the hole and visually inspecting it for arcing marks. Since holes are not reusable (the spring has sprung), best to move it to the side screws, and torque to spec with a torque screwdriver.
Torque screwdrivers are required as of NEC 2014 because of many, many failures of terminals that were hand-torqued by pro electricians - science has shown no one can hit torque specs "by feel".