Why is the ice on a frozen lake sometimes cracked?
Ice does crack completely naturally without human intervention. There are many reasons it could do so, one is as mentioned before mechanical stress from in- and outflow that lifts/settles the ice cover.

Another one is temperature stress from density changes, because ice changes its density e.g. depending on diurnal temperature changes or simply by cooling as it thickens (or warming as it thaws).

... and more.

Changing density:
https://www.engineeringtoolbox.com/ice-thermal-properties-d_576.html