Such a system is self-starting
Merriam Webster
capable of starting by oneself or itself
Or self-initiating
caused or facilitated to begin by oneself : initiated by oneself
The idea of critical mass is actually a little confusing in this question. If there is a critical mass, it is of something (fissile nuclear material, people exchanging ideas, reactive chemical constituents, etc) present in sufficient quantity to start a process (nuclear power generation or nuclear explosion, generation of new ideas, chemical reaction etc). The process is often described as a chain reaction.
Merriam Webster
chain reaction:
a number of events triggered by the same initial event
The chain reaction process started by a critical mass is therefore a only a special case of a self-starting process.
Self-starting does not relate to what happens after starting. The process may finish abruptly as in nuclear explosion; it may continue in a self-perpetuating manner as nuclear power generation or in a forest fire; it may proceed in some incremental way, with the addition of new material or ideas.
Continuous generation, explosion, incrementalism and self-perpetuation are therefore all secondary to the starting of the process and do not describe your key need for the gerundial adjective self-starting.