In this context, "go over the cliff" does not have its common meaning of going too far, taking a drastic step (though that could be a secondary meaning that it is playing with). "Cliff" is a technical term here, referring to an aspect of what is called vesting.

There are different kinds of vesting, but this refers to vesting stock options in employees: that is, employers giving employees the right to buy stock and thereby make a profit, as a form of remuneration:
http://en.wikipedia.org/wiki/Employee_stock_option

What happens with these schemes is that employees are given a certain amount of stock option periodically, say every month or every quarter. But with cliff vesting, which is common, there is a cliff, which means that for a certain period, commonly a year, after the employee joins the company, no options are vested, and then at the end of the year a whole lot is vested at once. After that, the normal periodic vesting (monthly, quarterly, or whatever) continues.

It's called a cliff because if you look at this on a graph, after the cliff it's a simple step by step rise, but before the cliff the graph says horizontal, at zero, for the first year (nothing being vested) and then suddenly goes up vertically, rather like a cliff.

So "go over the cliff", referring to what happens when you leave the company (or are dismissed) before the cliff date, is a kind of joke: by failing to reach the cliff date, it is as if you had fallen down to the ground; having had a prospect of options, you suddenly have none.

It really means "miss the cliff".

"In cases of partial vesting, a "vesting schedule" is a table or chart showing the portion of a right that is vested over time; typically the schedule provides for equal portions to vest on periodic vesting dates, usually once per day, month, quarter, or year, in stair-step fashion over the course of the vesting period. Often there is a cliff by which the first few steps in the graph are missing, so that there is no vesting at all for a period (usually six or twelve months in the case of employee equity), after which there is a cliff date upon which a large amount of vesting occurs all at once."
http://en.wikipedia.org/wiki/Vesting#Vesting_arrangements_an...

"One of the most exciting aspects of joining a startup is getting stock options. It gives you ownership in the company and aligns incentives between management and employees. However, one part of the standard options package causes a lot of debate amongst employees and management. It’s the Cliff.
A typical options vesting package spans four years with a one year cliff. A one year cliff means that you will not get any shares vested until the first anniversary of your start date. At the one year anniversary, you will have 25% of your shares vested. After that, vesting occurs monthly. So, if I’m a startup engineer granted 4,800 shares in my options package, at the one year mark, I get 1,200 shares vested (if I quit or am fired before that date, I get zero). After the one year mark, each month I stay with the company, I get another 100 shares vested (1/48th of the options package).
Many startup employees hate the one year cliff. Managers and VCs like it since they think employees will work really hard to make sure that they reach the cliff date. Employees, on the other hand worry that management will let them go just before they reach the cliff. The sad thing is that I have seen this occur at startups. You have an employee who is decent, but not great. Management keeps him or her for nearly a year, but then let’s them go a month before the cliff."
http://www.businessinsider.com/everything-you-need-to-know-a...

This passage shows that it's not necessarily a matter of the employee choosing to leave; sometimes they are fired just before the cliff date and lose their options. To use the same metaphor, they don't jump over the cliff, they are pushed.

"Vesting refers to the process by which an employee earns her shares over time. The most common form of vesting in Silicon Valley is monthly over four years with a one-year cliff. That means you earn the right to 1/48th of the shares you were originally granted per month over four years (48 months), but you don’t get anything if you leave prior to your one-year anniversary (and go over the cliff)."
https://blog.wealthfront.com/vesting-stock-options/