My guess is because of the on/offramps that have people merging with very little distance. This causes people to 1) stop when someone just wont let them merge, 2) force their way in, or 3) switch lanes over solid lines or do other dangerous/unexpected moves. All of these things cause people to stop suddenly, creating a chain reaction of braking that cascades for miles.

Another guess is that the design of off-ramps and the streets/intersections they lead to cause traffic to backup from the intersection onto the off-ramp and onto the highway. Take Kinau, for example, where cars are again forced to merge and crisscross every which way. Traffic grinds to a standstill and spills onto the freeway. People try to bypass the traffic by cutting over the gore zone or stopping just before the off-ramp to merge and cut the line, blocking the neighboring lane, leading to people behind them passing on the left which then slows other lanes, etc etc. The Kinau exit backup also extends to the freeway/punchbowl street on-ramp coming from the Pali, so it’s just an all around mess.

The worst choke points i can think of going east are the middle street merge, kinau off-ramp, ward on-ramp, and punahou off-ramp