Somebody hand Tanis another Molson ... as the prize for best answer for the "lumpy yolk" problem! It seems that when eggs are frozen, slightly frozen, or held for some time "near" freezing the yolk will gelatinize .... thus the most likely cause of the "lumpy" yolks.
Now we can look at another part of what Linda said ... (a) it was just "some" of the eggs that had this problem, and (b) "I never had this problem before." - and - "But I have had this problem several times."
My best guess here is where the eggs were stored in the 'fridge, the air flow within the 'fridge - and the temp. A carton of eggs stored in the coldest part of the 'fridge up against a wall that has an airflow that could cause a "wind chill factor" on one side of the carton .... that would account for only "some" of the eggs being affected. This also leads to the question - where in the 'fridge were the eggs stored when there was a problem and where when there wasn't a problem? One possible solition would be to put the eggs on the middle shelf and away from the sides or back. Of course, using a refrigerator thermometer and checking the temp in various locations would also be a great idea - that way you would know where the cold spots are.
The final part of the problem - the yolks breaking. It could be maybe from cold storage, age, or it could be technique - or a combination. Eggs cracked on a flat surface are less likely to have a problem with the yolk breaking than those cracked over the edge of something (side of a pan, bowl, etc.)
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain