The six year re-alignment period between the lunar line-of-apse and line-of-nodes is set by the planets.

Reference:

http://astroclimateconnection.blogspot.com.au/2013/10/connecting-planetary-periodicities-to.html

Given that:

DT = the lunar Draconic year ________=  0.9490 sidereal yrs = 346.620076 days

DP = lunar nodal precession _________= 18.599 sidereal yrs 

AT = the lunar Full Moon Cycle______= 1.1274 sidereal yrs = 411.784430 days

AP= lunar apsidal precession________ = 8.851 sidereal yrs  

AD = alignment period of the lunar line-of-apse and the lunar line-of-nodes = 5.9971 sidereal yrs

where     1 -- 1/AT = 1/AP ,       1/DT -- 1 = 1/DP* and  1/AD = 1/DP + 1/AP*** 

and

T_J = Sidereal orbital period of Jupiter __= 11.8622 sidereal yrs = 4332.75 days

S_{JS =} Synodic period of Jupiter/Saturn  _= 19.859 sidereal yrs               

S_VE = Synodic period of Venus/Earth__= 1.5987 sidereal yrs

It can be shown that the apsidal precession period of the lunar orbit is linked to the synodic periods of Venus/Earth and Jupiter/Saturn by the following relationship:

AP ≈ [S_JS×10S_{VE] / [}S_{JS +} 10S_{VE] = 8.857 yrs}

[with an error of 0.006 sidereal yrs = 2.2 days] 

and that the lunar nodal precession period is linked to the sidereal orbital period of Jupiter by:

5/4×DT = (1/10)×T_J**

See the following link:

http://astroclimateconnection.blogspot.com.au/2013/06/linking-orbital-configuration-of.html

Now this last equation can be rearranged using the relationships (*)  and (**) to give: 

DP = T_{J / [25/2  --} T_{J] = 18.599 yrs}

Hence, using the relationship (***), we can see that the six year re-alignment period between the lunar line-of-apse and the lunar line-of-nodes is synchronized with the synodic periods of Venus/Earth and Jupiter/Saturn and the orbital period of Jupiter.