1.) If you run out of heel by cutting a line wider/deeper than the heel is.
2.) Too small of a heel.
3.) Unpredictable cast metals, especially bronze and brass.
4.) Lack of control (operator error).
Engravers with more experience tend to react faster and recover before a graver dives too deeply, but it still happens to the pros.
There are several reasons this can happen and usually operator error is the culprit at my bench.
Can you explain what you mean by "digging in" a bit further? Do you mean the graver suddenly dips at a certain point in the cut and then comes out, or does it get stuck altogether?..........or maybe none of the above. A little more info may help narrow down the possibilities.
I recently posted a problem I had with an 1858 Remington replica by Pietta. The stainless steel (probably 300 series) was the very definition of "digging in." The surface was very hard for about a 1000th of an inch then soft under. This caused me to have to start my graver at a high angle to get a bite then once the point bit in the graver wanted to go to China. My hand would immediately react to corrrect then the graver wanted to come back out of the cut. This was all very frustrating. I have cut carbon steel Pietta parts that cut like butter so the "digging in" problem can be the metal if you don't normally have this problem on mild steel and precious metal.