Charles Ellicott Commentary Exodus 19:21

"And Jehovah said unto Moses, Go down, charge the people, lest they break through unto Jehovah to gaze, and many of them perish." — Exodus 19:21 (ASV)

Lest they break through — that is, force their way through the barrier which Moses had erected.

To gaze, and many of them perish. — Some might have perished by the execution of the orders given in Exodus 19:13. But the allusion is perhaps rather to such a heaven-sent plague as destroyed the men of Beth-shemesh to the number of 50,070 (1 Samuel 6:19).