They are more Emotional than Humans
Intelligence is only one factor when considering how a person (or orc) will behave. Hormones play a major factor as well. Despite being an intelligent species, humans quite often make very reckless choices because of the feelings we have. Picture the difference between a woman's aggression and patience at different points in her natural cycle. Now picture PMS as being the stable end of an orc's natural cycle. Then try to picture a state significantly amplified to represent a female orc's version of PMS, and a male orc's normal state being somewhere in between these two points.
Although orcs are able to reason out that the humans near their borders are not worth attacking, their gut feelings tell a different story. When a human feels slightly offended, an orc feels mortified. When a human feels aggravation, and orc feels rage. This gives orcs a very smash first, think later nature.
Hormones Impact Etiquette
Because of these strong emotions, orc society has developed such that all orcs are extremely considerate of each other and have developed a very high standard of empathy to keep from triggering one another. They have very strict, almost ritualistic rules of manners to make sure they are not constantly offending each other and starting fights. Orcs always knock, and wait 3 seconds before entering a room. They always raise their hands before they speak to make sure they do not interrupt a conversation. They always use "I feel" instead of "You make me feel" to prevent blame escalation. And if an orc sits down during a debate, it is because he/she needs time to collect their emotions so all other orcs in the room stop talking until the upset orc is recollected. And most importantly, they always send their families and neighbors well thought out gifts on important holidays to make sure everyone feels known and important.
This Makes War Practically Unavoidable
But those rude, heartless humans... they just walk right up to you and start talking, they interrupt, they never stop to ask how your feel, they are condescending. But what really makes things difficult: they never EVEN ONCE remembered to send the orcs a proper Autumn Equinox gift. A good orc knows that the Autumn Equinox is when you visit your neighbor tribes to make sure everyone has enough food stored for the winter, but the humans never check on the orcs, and when the orcs try to check on the humans, they just lock their gates and start yelling and this makes orcs angry. Angry enough to want to smash in the gates and start slaughtering everyone inside.
Their land is not the Utopia the orcs have, and is in fact resource-scarce.
This makes the situation even worse. If the orc lands are rich in resources, and the human lands are poor in natural resources as described, then this powder keg may in fact have fusses burning from both ends. Humans go to war over resources all the time. If the orcs have them and the humans do not, then the humans will likely feel compelled to take those resources by force. Humans don't check on neighboring kingdoms to make sure they have enough food for winter, they just pillage each other if they don't. So, between the humans offending the orcs, and the orcs having what the humans want, war is pretty much inevitable.
Once Started, the War Can Not Be Ended
The lands surrounding them are peaceful, but offer a fierce resistance ... They do not provoke the orcs, but are capable of defending themselves.
The humans of this generation may have learned a long time ago not to provoke orcs, but that does not mean that humans in the past knew better.
If an orcish parent tells her children about how hundreds of years ago humans attacked the orclands, the children take this VERY personally, because that is how their emotions work. This maddening hate for humans could last for generations, perhaps even growing worse over time as each telling of the stories is amplified by the disdain of each new generation; so, even if the orcs successfully drive the humans away once, when the humans return 200 or 300 years later with peaceful intentions, the sins of their forefathers are not forgotten, and certainly not forgiven. While the humans see the war with the orcs as a footnote in history; to the orcs, humans are and forever will be bitter enemies.