Exotherm, and the runaway
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The loop
The reaction is exothermic. The heat it releases raises the temperature of the mass, and the reaction rate rises with temperature, releasing heat faster still. In a thin layer the heat escapes and nothing happens. In a deep mass it cannot escape and the loop closes.
What a runaway looks like
Smoke, a sharp smell, a cup too hot to hold, resin that yellows or cracks, and in bad cases a container that deforms or melts. It happens over minutes and it is not recoverable once it starts.
Which is why deep pours are a different product
General purpose resins specify a maximum pour depth, and deep pour resins exist as a separate formulation precisely because of this. Using a standard resin for a thick casting is the classic way to find the limit, and the limit is on the data sheet.
The container is part of the thermal problem
An insulating container holds the heat in. A tall narrow cup concentrates the mass and the heat; a wide shallow one spreads both. This is one of the few crafts in the library where the container actively changes the chemistry rather than simply holding the result.
Ambient temperature moves everything
A warm room shortens pot life and raises the peak; a cold one can leave a cure incomplete. The same measured mix behaves differently in July and January, which is why experienced makers record the room temperature alongside the batch.
If a cup starts to go
Get it outside, onto something non-combustible, away from anything that burns, and leave it. Do not try to pour it or carry it to a sink. It is making its own heat and nothing you do stops the reaction.