Why is there grease coming from my SDS chuck?

Grease or oil coming only from the chuck is normal, as it is a masonry grease applied to the SDS chuck to reduce the friction of the moving components.

This is usually a high-temp grease, so leakage like this may indicate the chuck is being worked a bit hard or for extended periods.

You should clean and apply grease to SDS+ fittings regularly, to maximise the life to the fitting and the SDS Chuck.

This grease is an example:
KG-50G