It looks like some people were simultaneously banned following that thar' flamewar (a bunch of posters disappeared at the same time, and there's chatter) so d(users)/dt can be negative.
d(users)/dt is growth in users, not their number. To illustrate: if 600 users registered in February, then only 400 in March HN's growth would be negative; d(users)/dt would be negative.
d(users)/dt would be the rate of change of users. That can only be negative if the amount of users decreases, ie through bannings.
You're thinking of the second derivative, d^2(users)/dt^2, which is the rate of change of the rate of increase of users. If that is negative, it means that the growth has slowed down, as in your example with February and March.