What causes implant to move towards the armpits? How can this be prevented? 3 weeks post op.

Implant displacement is a well known undesirable outcome of some breast augmentations.  It can happen when implants are properly selected and not enough support is given to the breasts postoperatively.  Also, when choosing an implant size some patients actually request or do not mind when the implant is shifted towards the side of the chest ("side boob as they call it").  This means that the width of the implant is excessive.  This may be due to the desire to have a wide implant creating both cleavage and some implant show along the side.  When the implants are placed above the muscle, there is less tissue to prevent this happening. 
In my experience, a properly chosen implant along with a good post operative bra can prevent this if it is your desire not to have this happen.  I tend to like Caromed bras because they are very stabilizing and keep the implant going in directions you want them to go and prevent them going in directions you don't want them to go. A nice form fitting bra can be very useful in preventing unwanted displacement of the implant.
Surgically, over the years I have tended to make the pocket only as large as it needs to be for the implant to be inserted and remain in its position without folds. My desire is that the implant stays in the desired site without 
When you revise these complications, the pocket for the implant is much larger than the implant itself.  This allows the implant to shift considerably making the excessive pocket a fixed and permanent issue.  The surgical correction is to tighten up the sides of the pocket so that the implant remains more centrally located.
So, the main point here is that you have to maintain the position of the implant with a properly fitting bra (preferably prescribed by your Plastic Surgeon).  If it does displace, the surgical correction is not difficult.

The movement of breast implants towards the armpits could be due to factors like implant size, pocket creation during surgery, or inadequate support. It’s crucial to follow post-operative instructions, wear supportive bras, and avoid strenuous activities. If concerned, consult your surgeon for recommendations based on your situation.

There are three things that can add up to implants falling to the sides more over time - your own tissue strength, your chest wall angle, and the dimensions of the implant itself.  Exercise, even when subpectoral, should not change the position.  If you have poor tissues that have lost their elastic recoil, or you have a chest wall that causes your breasts to sit to the sides naturally, then you may be at a higher risk of them migrating to the sides over time.  I hope this helps.