Expiry of their old licence to be precise, now that Licence and Spectrum validity are no longer synonyms.
It was pre planned, no operator is foolish enough to make a price commitment for an actual lifetime when inflation rate in the market is through the roof.
They launched the lifetime incoming validity offer towards the end of their 20 year licence period, knowing that the unsuspecting subscribers will fall for it and that they will have to honour it only for a few years and gain subscriber additions in return.
Their licence expired in 2014/15 whereas Jio commercial launch happened in 2016. They just continued the incoming validity beyond the licence expiry without any obligation, but Jio's entry just triggered them to end it sooner than they had planned. They needed to increase revenue to be able to invest in network since Jio's network was built for data while theirs was built for voice, so they were bound to lose the data subscriber war like all the smaller players did.