I've found improper valve adjustment has caused rockers to break in BMW motors but I have no experience with Nissan.
Here is something from a google search.
Rocker Breakage!
– What Typically Causes It – Too Much Spring Pressure, Bad Geometry, Wild Camshafts?
Factory rocker arms were designed to run a long time in the atmosphere of a stock engine. Subjecting them to increased spring pressures or aggressive cam profiles can be a disaster. Trunion bearings, axles, nose wheels, and pushrod cups can fail at any given time. Quality aftermarket roller rockers are designed with high performance hazards figured into the equation to help prevent these failures.
Aftermarket roller rockers can be, manufactured from a variety of materials. Before choosing a particular rocker package, one should have a complete understanding of the duty requirements of the rocker arms. The lower cost, lighter duty kits were designed to replace stock or milder cam and spring combinations. They are often manufactured from low cost, cast aluminum. They will not stand up to the rigors of stiff valve springs or extreme camshaft ramp speeds. The higher end kits which are made from high quality billet aluminum like 2025 t6 for instance, can withstand incredibly aggressive cam profiles and very high spring pressures. This particular alloy actually becomes stronger as the temperature rises. Steel is often used in the most expensive rocker arm kits, but it too can be subject to failure. When a steel rocker arm breaks, it is usually in the area of the trunions or trunion bearings. An oil pan full of needle bearings is a bad thing. While aluminum is not as strong as steel, it has some flexibility which allows the rocker body to take much of the punishment rather than the trunion.
Extreme camshaft ramp speeds cause the lifter to have difficulty maintaining contact with the camshaft lobe.
This condition is often referred to as "valve float". Aggressive or poor cam lobe designs or opening ramps with high acceleration rates can literally hammer the rocker arms and other parts of the valvetrain. To help control this condition, engine builders usually move up to stiffer valve springs. While this tends to solve the immediate problem, other issues generally appear. Exceeding the manufacturer’s recommend / allowable spring pressure is a sure way to kill the rocker arms. When in doubt, step up to the next level rocker arm. Doing the job twice is always painful. Determining which rocker to use for your particular application is the responsibility of the engine builder.
High pressure valve springs cause sever loads on the pushrods. These loads imparted into the pushrod cause it to bend and then return to a straight configuration. This unloads a sharp energy pulse to the rocker arm, which transfers it into the valve/valve spring assembly. Continued abuse can cause the rocker arm to fail. A better choice would be to utilize lighter valves, lifters, retainers, lighter / stiffer pushrods and moderately stronger springs.
Improper rocker arm geometry can place the rocker in disadvantageous positions which cause severe loading to the rocker. Too much of this, and the rocker arm can fail.
Insufficient clearances can subject the rocker arm to contact with other immovable objects. Coil bind, piston to valve clearance, retainer to seal clearance, etc. can force sever loads on the rocker arms that are not part of manufacturing concepts. The result, of course, is rocker failure.
One final cause of rocker demise is poor engine oiling. This can occur for a variety of reasons. Often overlooked areas that can cause serious oil pressure loss are the cam bearings and the main bearings. Improper / loose specs here allow a great deal of oil to escape and never reach the upper end and rockers. Poor oiling starves the trunion bearings, axles and nose wheels. Add aggressive cam profiles and high spring pressures to the equation and rocker arm failure is imminent.
