Three factors for this that I'm aware of:

1. The whole road system needs to be able to handle the throughput from all sources to all destinations in aggregate. That means if you relieve one bottleneck, it increases the rate that traffic can reach the roads on the other side of the former bottleneck, which can cause them to exceed capacity. Eg, if you have a 5 lane highway reduce to 3 lanes and jams frequently form at (and leading up to) the lane reduction sites, traffic will likely flow ok in those 3 lanes and then really open up if it goes to 4. But if they extend the 5 lanes to where it goes to 4, there will now likely be a jam where traffic used to flow decently where it was 3 lanes, and that "really opens up" also gets downgraded to "flows decently".
2. Intersections are horrible once the exceed capacity, especially if there's any potential for starvation in the way they cycle, as that can result in not just a jam but grid lock. Starvation in this context means a vehicle never gets their turn to proceed through the intersection in the way they desire. People waiting for their turn that will never come back up as more queue up and eventually start blocking the other paths through the intersection and other intersections leading there start also seeing starvation and eventually you need a person to go direct traffic to get things flowing again. Grid lock is when all paths get starved and will just continue backing up until everyone who wants to go to the gridlocked area is stuck in traffic that isn't moving at all. It's also why it's illegal to stop inside an intersection because then you start starving any lanes you block. Anyways, adding a new road or lanes can suddenly cause an intersection downroad to exceed capacity.
3. More onramps/offramps on a highway increase turbulence as people need to change lanes, especially in those cases where the lane you enter on immediately exits, as you get both people wanting to join and leave that lane in a short area. Turbulence creates traffic waves as traffic alternates between slowing down as people are forced to brake for lane changers having trouble moving over, and the slowdown effectively increases the road capacity (ahead of it, behind it has effective reduced capacity), so those at the front can immediately speed up, but since these affect different lanes at different rates, the lane weavers try to move to the fastest lanes and create more turbulence, sometimes it all adds up to bring some of the traffic to a complete stop. The same thing happens when people try to skip the queue and force their way into the backed up lane at the last moment.

Asshole drivers make bad traffic situations way worse than they need to be. But road systems are a lot more complex than they seem to be. It's like fluid dynamics but even more complex because each driver has a different will (and agression, competence, etc) and it's several ton vehicles that break if they touch each other acting as the basic particles instead of particles that usually move past each other relatively easily.

Induced demand.

I dont think so, people just tailgate too much so they cant handle a little traffic, just go the long way and skip everyone else in the conga line not realizing a conga line moves very slowly.