  The short squeeze of GameStop (GME) shares in mid-January 2021 has been
primarily orchestrated by retail investors of the Reddit r/wallstreetbets
community. As such, it represents a paramount example of collective
coordination action on social media, resulting in large-scale consensus
formation and significant market impact. In this work we characterise the
structure and time evolution of Reddit conversation data, showing that the
occurrence and sentiment of GME-related comments (representing how much users
are engaged with GME) increased significantly much before the short squeeze
actually took place. Taking inspiration from these early warnings as well as
evidence from previous literature, we introduce a model of opinion dynamics
where user engagement can trigger a self-reinforcing mechanism leading to the
emergence of consensus, which in this particular case is associated to the
success of the short squeeze operation. Analytical solutions and model
simulations on interaction networks of Reddit users feature a phase transition
from heterogeneous to homogeneous opinions as engagement grows, which we
qualitatively compare to the sudden hike of GME stock price. Although the model
cannot be validated with available data, it offers a possible and minimal
interpretation for the increasingly important phenomenon of self-organized
collective actions taking place on social networks.
