I've had this problem, too.
Things that have worked for me:
1) check your border seams to make sure that they are not slightly puckered. Puckering adds fullness to the edge of the border (like a ruffle), thus producing wavy borders. This kind of puckering is caused by the thread tension being too tight.
2) Suggestions to square the quilt before adding border are good. Of course, now that means taking the borders off and then squaring the quilt. If you do that, I will suggest that you:
A) spray the quilt top with water, put it in a lingerie bag and toss it into the dryer to return all blocks to their original shape. I think you will have to do this because it looks like some of the squares are stretched.
B) Do the same with the border strips you have sewn on.
C) If you want to press the quilt top and the border strips after taking them out of the dryer, make sure you PRESS, not iron. Ironing will stretch the bias. Many quilters use starch to stiffen the fabric and make it less stretchy. I can't do that because it sets off my allergies, but, based on who coached me through it and seeing their quilts, it works.
D) Measure the quilt 3 times horizontally and vertically. Horizontal measure top, middle, bottom. Vertical, do the same, the left, middle, right. Calculate the average of horizontal, calculate the average of vertical measurements. Cut your borders the average length of horizontal and vertical.
E) Pin the left and right borders on. Start sewing from the center of the border to one end. Then go to the center start point and sew to the other end. Make sure not to PRESS the seams.
F) Pin the top and bottom borders on. Again, start sewing at the center of the border and sew to one end, then return to the center and sew to the other end of the border.
G) repeat number 2 for each border.
I sincerely hope this helps. It seems like a lot to do, and it is, but it gets to be a habit and part of the process over time.