New Zealand Rugby Foundation chief executive Lisa Kingi-Bon said after his admission to hospital, Anderson suffered a fat embolism - a complication where fat, sometimes from bone marrow, leaks into the blood system.
In rare cases it can cause respiratory problems and lead to a coma, or death.
To compound matters Anderson was then struck by an unrelated lung infection, and his condition became critical.
Three days after being admitted to hospital, the young rugby player was placed on life support.
"It was just a completely freak situation," Kingi-Bon told the Herald on Sunday.
"It was a freak development. Just out of left field. It's been very touch and go."
Anderson spent a week in the intensive care unit before "turning a corner" and being transferred to the high dependency unit last week.
On Thursday he was again transferred, to the orthopedic ward, and was awake and talking to family who had flown from Perth to be at his bedside.
The foundation supports catastrophically injured players and has been helping Anderson's family since he was hospitalised.
Anderson, who has eight siblings, had not had a family member leave his side.
"It was very scary for all of them," Kingi-Bon said.
Anderson plays club rugby for Te Puke but was picked up by Thames Valley as a loan player for the Heartland Championship. He was earlier selected for the Heartland XV in 2015.
He had just returned to New Zealand after a stint with the Australia Under 20s and Perth Spirit, and last year was named for the Chiefs development squad.
Family declined to speak but told the foundation that they had had to explain to Anderson what had happened to him.
On Thursday he was awake and able to play cards with his family, and on Friday he posted a video of his career highlights to Facebook, prompting well wishes from his supporters.
Given his age and how fit Anderson was, his condition had shocked everyone, Kingi-Bon said.
"Everyone was extremely worried, but there's just a great outcome and we're just really relieved and really delighted that he's turned a corner firmly.
"I've never seen anything like it in the five years I've been with the foundation."
He had a long road of rehabilitation ahead of him and would spend the foreseeable future - at least some weeks - in hospital recuperating.
Te Puke coach Craig Jeffries, who had been a frequent visitor to Anderson's bedside, said he was a "much loved" member of the rugby community.
"His injury and his battle had a massive effect on all of us. We are so relieved that he is recovering well. He's a tough bugger and we know that his resilience will stand him in good stead."
In his two years with the club Anderson had transferred from loose to the front row and Jeffries said his transformation during his time with the team had been "immense".
Thames Valley Rugby Union's chief executive Edmond Leahy said the bulk of the Thames Valley squad had met Anderson just a week before the game.
"However the whole team and everyone who knows Hayden from Thames Valley has been concerned about his condition and wish him a swift and successful recovery."
