Not sure why you didn`t see it coming, he had one year left on his contract and big money due against the cap this year makes sense. Peters is still an upper echelon tackle, I doubt he sees all 5 years, I am hearing 19 mill guaranteed which basically makes it a 3 year deal.
I didn't see it coming because he is old. He is 32 and will be 33 next year when his contract ends. It's a risk. But, I guess they decided they would roll with him until he is about 35. The contract apparently has $19 mil guaranteed, so it seems like the back years are voidable. All in all a decent deal.
Considering he missed so much time with the Achillies injury he is not an old 32 and lets not forget Tackle can play well into their thirties, just look at Runyan and Thomas. I think he is here 3 more years.
Also keep in mind that these financial terms are coming from the agent. The $19.55M "guarantee" isn't necessarily fully guaranteed. Sometimes deals are guaranteed in case of injury but not for "skill". That means if he gets hurt he gets paid but if they just cut him because of declining performance he doesn't make as much.
The $19.55M "guarantee" is really only $9.55M of additional guaranteed money. He was already due $10M this year and while it might not have been technically guaranteed it was practically because we weren't cutting him. So it's really an additional 4 year, $41M deal with a little less than $10M in guaranteed money.