What would it take to dislodge or damage a hair graft 14 days after a hair transplant?

Typically at this stage the only thing you should be concerned about would be a bad sunburn during the first 90 days. Otherwise the grafts are pretty resilient if the surgery was performed adequately.

If you really want to damage the transplanted grafts after 14 days, then you must try very hard.  Otherwise based on the study, it should be secure.

Anything that can damage the
scalp can potentially damage the newly transplanted grafts, i.e. severe sunburns, trauma, etc.

Great question! Very little can damage grafts at this point, but keep in mind the study did not take into account factors such as traumatic injury that causes significant bleeding (hitting your head on the kitchen cabinet or car door). It is unlikely to cause damage but this has never been addressed.All in all, at 14 days grafts are quite secure and the risk for loss from most external forces is extremely low.

It is important to be careful for the first week after hair transplant to make sure they don't dislodge. However after two weeks, they should have settled. 

I am one of the authors on that paper with Dr, Bernstein. Once the crusts are gone, the grafts are secure and if you pass the 14th day with no crusting, no problem. It would be very difficult to get the grafts out. I have put a copy of the article in the web reference below for others to read which is alo fouond on my website.  See below.