Can B2901/02/11/12A connect its output to the ground when the output is switched off?

Yes, the B2901/02/11/12A can connect its output to the ground by specifying the output-off state to "ZERO" when the output is switched off.
The B2901/02/11/12A supports three output-off states, which are "NORMAL", "ZERO" and "HIGH Z".
The output-off state is the state when the output switch of the channel is set to off.
If you set its output-off state to "ZERO", the output relay of the channel is still close and source 0 V after the output is switched off.

NORMAL is the default setting.
In this output-off state setting, the output relay of the channel is open and the source and range settings are changed according to the pre-defined rule.

If you select ZERO as the output-off state, the output relay of the channel is still close. And the source and range settings are changed according to the pre-defined rule.

If you select HIGH Z as the output-off state, the output relay of the channel is open and the source and range settings are kept.
However, this setting is not available if the present range setting is 1 A range or more.

Their detailed states are summarized in its User's Guide below.
http://cp.literature.agilent.com/litweb/pdf/B2910-90010.pdf