As a preventive 'fix', the base depth can be
increased by making an upside-down T that
ties front and rear legs together at floor, but
extends past legs.
Consider torque. Join at sides.
See pictures at:
http://woodworkbenches.net/choosing-the-best-workbench-designs/
and
http://16kwoodworkingplans.com/basic-woodwork-project-the-workbench/
-J