https://en.m.wikipedia.org/wiki/2,4-Dichlorophenoxyacetic_ac... https://en.m.wikipedia.org/wiki/Dicamba https://en.m.wikipedia.org/wiki/Mecoprop

These are the three most common in consumer broadleaf herbicide ingredients (usually combined) that actually work. They mimic a growth hormone:

https://en.m.wikipedia.org/wiki/Auxin

Between application time and death the plants really do just grow out of control in weird uncoordinated ways. You can see a treated dandelion here:

https://turf.purdue.edu/wp-content/uploads/2015/03/Slide16.J...

Another interesting broadleaf herbicide mechanism is overloading invasive plants with chelated iron. Broadleaf weeds absorb iron rapidly and without regulation until the point of toxicity, whereas grasses will only absorb what they need to turn a nice healthy dark shade of green.