Beginning in 2006 and going through 2016 BYU appeared in every National/Varsity Cup Championship game (almost always against Cal). They won their first championship in 2009 and then 4 straight from 2012 - 2015. In 2016 they played Cal at a packed South Field, were heavily favored, and had a 29-19 lead with 20 minutes to go but could not respond to late substitutions by Cal and got run off the field in the last 20 minutes loosing 40-29. They have not reached a final since then and shortly after that the coaches all left to UVU (with absolutely no subsequent success at UVU). BYU's rugby team is now led by Steve St. Pierre (who I'm pretty sure played on and may have captained the 2009 National Championship team). They seem to be improving again and I'm actually looking forward to the game this weekend against Cal at South Field (and glad there isn't any rain in the forecast either).

That history is pretty straightforward, but your question of why the rapid decline from 2016 is more nuanced and likely you will get different answers from different people depending on their individual perspectives (just like the story of the 5 blind men each describing an elephant but each only actually describing the thick leg, massive body, thin and floppy ear, the long flexible trunk or the tusks). So here is my perspective:

The rise of BYU rugby beginning in the early 2000s and peaking in the 2012-2015 years directly corresponds to the extremely successful U19 Club teams of Highland (Salt Lake County) and then United (Northern Utah County). Highland dominated high school rugby for something like 20 years beginning in the 1990s by bringing import players (mostly from the South Pacific) to create the best program in the country. United then followed that same model and challenged Highland in 4 consecutive National Championship games beginning in 2008 (I believe) and winning one in 2010 in a fantastic game at Rio Tinto (https://www.youtube.com/watch?v=m4562PtLtp8).

USA Rugby eliminated the U19 designation in 2011 and made it basically impossible to import players to high school clubs. Highland rapidly disappeared from the national high school scene (its founder also went on a mission which didn't help). United continued its run for a couple of more years with a great core of local players and it would play in the national championship games in both 2012 and 13 (winning in 13) but after 2015 United declined significantly and was no longer nationally relevant. Herriman also emerged for a time as a fantastic high school club (ranked 2nd in the single school division in 13).

If you look at the core rosters of the BYU Championship run it was filled with players from Highland and United and specifically from that 2010 national championship game (who had come home from missions for the 2012 season) as well as a couple of All Americans from the United 2013 championship team. However, Utah is no longer the undisputed king of high school rugby and consequently BYU Rugby does not have this huge disparity of talent to draw on as compared to the rest of the country as it did.

Yes, there were challenges and conflicts between the BYU coaches and BYU administration which made it more difficult for BYU rugby.

Also talented and capable assistant coaches moved on (or left) which also hurt.

However, even with those factors, the decline of BYU Rugby was directly tied to the poor player development during the 2012-16 time and the then coaching staff's inability to recognize the changing landscape around them (i.e., that the Utah/LDS rugby talent was not nationally dominant as it had been). I have spoken to several former BYU players who started for multiple championship seasons at BYU they did not have great praise for the BYU coaches that left and went to UVU.

In my opinion, given poor state of the program in 2017 and 18, the best thing that happened to BYU rugby was the staff's mass exodus to UVU requiring a reboot and rebuild of the program.

So here's hoping that Steve St. Pierre and his staff can continue building a solid program at BYU and that they beat Cal this weekend.