Yes, that, or bad solders.
Tin solder has a lot more resistance than the wires themselves, and this is a problem with connectors that is tricky to solder with a good contact between wire and connector (deans, for example).
The deans connector itself might be taken way past 100A without any problems, if you got a good solder.
But actually getting a good solder on a deans style T is easier said than done.
Having a higher cross section on the connector than on the wires themselves makes little sense.
Which in fact means that you can go quite extreme with just a humble 3.5mm bullett connector. And it works, as long as toe solder is good.
Putting in 8mm bullets, which I see someone do makes no sense unless you are pulling 1000A. My tip to those using that is just: learn to solder.