1) Article 14 of the state constitution says the Legislature has "plenary power, unlimited by any provision of this Constitution to create and enforce a complete system of workers' compensation, by appropriate legislation", including things like workplace safety and medical care for injured workers. Prop 22 restricts the Legislature from providing some of these things to app-based workers, which SEIU argues conflicts with that provision of the constitution. So that part of Prop 22 is invalid, and by Prop 22's own terms that means the entire measure is invalidated.
2) Prop 22's definition of what constitutes an "amendment" to Prop 22 (and therefore can't be passed by the Legislature unless it furthers the purposes of Prop 22 and passes with a 7/8 vote) is unduly broad. It covers topics that Prop 22 doesn't itself legislate on at all, like collective bargaining or differential treatment of companies based on whether they classify their workers as employees or contactors. A ballot prop can't (other than by amending the constitution) restrict the Legislature's ability to pass laws that don't directly conflict with the prop, but Prop 22 tries to do that anyway by classifying them as "amendments". If this is allowed to stand, future props could similarly tie the Legislature's hands or impose supermajority vote requirements on entire policy areas, and that's not how things are supposed to work.