Actually, there were no units sold at all. No option exercises, nothing.
There are tax reasons why partnerships, including MLPs, stay away from option based compensation. Instead, what most of them have, including NRP, are phantom stock/unit plans. Under these plans, the MLP agrees to pay a cash bonus to an employee or director equal to the value of a fixed number of units on a particular day. Say you're a director. The plans says that NRP will pay you, in cash, an amount equal to the value of 1,000 NRP units on February 15, 2015, 2016 and 2017, assuming you are still employed by NRP on those dates (if you quit in the meantime, you lose the bonuses). If the units trade at $ 20 on those days, you will get a cash bonus of $ 20,000 on each of those dates. If the units go up in value to $ 40, the bonus will be $ 40,000, No units are issued; that is why they are called phantom plans. The bonus is measured by the value of the units, but no actual units are involved.
For SEC ownership rules, the vesting of the phantom units is deemed to be a buy (even though no units were issued) and the cash payment by NRP is treated as a deemed sale (again, even though the employee didn't sell anything). All the Forms 4 tell you is that NRP paid out bonuses of several million dollars related to 2013 performance.
ALL February insider selling is options exercised (or bonus shares) that were exercised and sold. Company officers or board members usually notify the sec months in advance of these sells. Happens with almost all companies. Does however look bad when the shares are selling at a 14 year low (check basic chart).