King Peter I had incidents with Austo-Hungary way before the war and the assasination of Franz Ferdinand. Around the end of 1905. Austo-Hungary was mad because Serbia and Bulgaria made a trading pact which now kicked Austro-Hungary from the no. 1 place in Serbian trading, they wanted Serbia and Bulgaria to withdraw that agreement but Serbia said "Lol no", and after that A-H banned all trade with Serbia, both import and export, fun fact is that Serbia exported almost 90% of livestock (Pigs mainly) to A-H, thus starting Customs war (aka Pig war) that lasted up to, 1911. if Im not mistaken, when new trading act was put in action.

However after the annexion (and prior to it) there were some political games played by A-H goverment which were met with disaprovement by Serbian populace, but this is too long to explain here. Perer I didnt have any influence or connection with Gavrilo, and at the time of the assasination he wasnt the "ruling king", on 24th of June he gave the ruling rights to his son Alexander I, assasination itself happened on 28th June and war started a month after, 28th July. Peter I after declaration of war went to the front and later withdrew across the Albania with Serbian people in 1915.

Its a very shallow explanation if Peter I political doings with A-H, there were many more sides in this political dance, both foreign and internal.