First, I need to correct some statements in your question.
India has always been wary of partnership with the US since the US threatened India during the 1971 war. However, the US acknowledged that it wasn't ready for a partnership with India then, but now it is "able and willing" to partner with India.
This is an absolutely incorrect and false statement.
In 1947, the USA invited India to join the USA's pole. However, India, because of Jawaharlal Nehru's personal decision, decided to keep itself non-aligned. The USA was also fully content with India's non-aligned role as India was a democracy and a virtual counterbalance to communist China.
In 1965, when Pakistan invaded India, the USA protected India by not allowing SEATO and CENTO members to get themselves involved. The USA also stopped military and monetary aid to Pakistan overnight.
In 1971, the CIA knew very well that the Soviet KGB was supplying arms and training to separatists (Awami League and Mukti Bahini). However, the CIA kept absolutely silent.
Therefore, as you can see, the USA always had an affection for India, but India, for some reason, decided to keep itself aloof.
India is also gradually moving away from Russia, especially by decreasing purchases of Russian arms and fulfilling the demand for indigenous, French, and Israeli weapons.
India is not moving away from Russia. India is trying to establish itself as an independent regional power with an independent foreign policy.
In line with that, former President Donald Trump stopped all defense and security assistance to Pakistan, calling out their lies on counter-terrorism operations (which has always been India's position).
This was not the case.
After a brief verbal spat, Pakistani PM Imran Khan visited the USA, where he was given a very warm welcome by Trump. The USA agreed to blacklist BLA as a terrorist organization, and some military supply deals were also signed. These deals were later postponed or denied because of Indian objections, Pakistan’s CPEC deals with China, and the nonprovidence of a CIA airfield in Pakistan through which the CIA wanted to keep an eye on Afghanistan.
The answer to the question:
My question is, what has changed now? Is it a punishment for India to buy Russian oil to combat inflation? Is it a reward for alleged Pakistani cooperation in killing Al Zawahiri? Is it the US's concern about not letting the Pakistan army get weaker to protect the nuclear arsenal? I see a lot of possibilities here, but what exactly is the reason for this renewed cooperation?
The change is because of a lot of things.
The Pakistan Army removed Imran Khan, allegedly at the behest of the USA, and the new government led by PM Shahbaz Sharif suspended many Chinese projects. He is also in talks with China about dismantling the CPEC authority. Therefore, the USA owes Pakistan some reward.
Pakistan has three requirements over which the USA has control: (1) clearance from the FATF gray list, (2) IMF monetary aid, and (3) spare parts and engines for F-16s.
#1 and #2 are self-explanatory.
Regarding #3, Pakistan will most likely replace all its US-made weapons with Chinese ones. However, it will take at least one more decade for China to catch up with advanced US technology. In addition to that, right now, Pakistan’s economy is not in very good shape. So, either way, keeping existing F-16s flightworthy is beneficial for Pakistan.
However, if the USA doesn’t supply F-16 spare parts and engines, Pakistan will buy more J10C from China. In the meantime, Pakistan will continue to provide China with F-16 information so that J10C can be customized according to Pakistan’s needs (which they also did in the case of JF-17s).
India's buying of Russian oil and arms could be another factor, but I don't think it is very important.
How will it affect India-US ties?
Nothing. This deal is not a ground-breaking one. It's a minor deal.
Note: I wrote this answer entirely from my memory. Please let me know if you need a reference and citation for any specific information or argument.