The signing bonus usually is intended to cover two things. (1) Incentivize you to go through the hassle of switching, getting used to a new place, learning new rules, new people, etc (2) Compensate you for the loss of whatever portion of this year's bonus you'd be giving up by leaving mid-year, and cover the loss of any un-vested benefits.
In subsequent years, neither reason still holds. You should now be eligible for the full year's performance bonus at your new employer, and the vesting period of benefits is intended to keep you there.