I believe you'll find that those keyword phrases were broadmatched to more expensive phrases during the change. This has been happening for years but it wasn't obvious because before the only indication would be that the average position for a few phrases would drop suddenly. Now their system calls attention to them.
You can confirm this by going to the keyword level, selecting "edit keywords" and "estimate traffic" - then click "find alternatives" for the keyword phrase with the high minimum. All of the words in the left column are broadmatched to that phrase. You should find your answer there.
I've been posting about this for a long time. It is getting to be more and more of an issue and it is, in my words, "strangling the Golden Goose". I have a thread elsewhere with examples and more detail for those interested.
The better your CTRs and the longer you've been advertising the more likely your keywords were broad matched. This matching appears to happen about once a quarter.
When really effective niche keyword phrases get matched some will see an increase in traffic and a reduction in conversions because ads are no longer targeted and may not even be relevant if you don't actually offer what has been matched.
For the advertisers who originally targeted the phrase where you originally had almost no competition you now have every ad appearing for the more general keyword phrase. I can provide specific examples or link to online information where I have already done so if the moderators permit.
I'm seeing tons of never seen before competitors for some niche keywords. These are ads are only related - not very relevant, mind you.
But the funny thing is that the keywords are supposed to be "phrase match" , not broad match keywords...
