What are the catalysts of change, both externally and internally?

There are different approaches to the question of social change and whether the catalyst to this change may be attributed to the properties of actors and their actions or to social factors—mechanisms that are external to these actors.
Theories that focus on the properties of actors (subjective factors) are "actionist" paradigms in which social change is the product of "individual actions." On the other hand, theories that emphasize causes, processes, and structures of a collective nature adopt a "holistic paradigm" in which these factors are external to individual actors.
Further, approaches that emphasize the catalyst of change in subjective factors seek to understand internal beliefs first; understanding these beliefs then explains social change (for example, Max Weber’s interpretive sociology). In contrast, approaches that focus on external factors seek to explain social change in the collective dimensions of social life. In identifying these objective forms of the social, it is then possible to understand subjective factors and changes in meaning (e.g. Emile Durkheim’s social facts).
Both these approaches lead to problems due to their inherent limitations. There are attempts to resolve these limitations through models that combine individual and collective factors as part of one process (e.g. Anthony Giddens’ structuration theory) and others aim to maintain their distinction to study their interplay over time (e.g. Margaret Archer’s analytical dualism).
Further reading:
Sociological Theory by George Ritzer & Jeff Stepnisky