I think it's and under load not overload.
A double charge is unlikely with the dillon according to a few members. But and under load is possible, a jam or obstruction of the powder and there is just a little powder in the shell.
And an under load is often more dangerous than overload. The ignition is to slow ending in extreme detonation.

The shock wave of such a detonation can cause other powder to ignite to and the problem is complete