In early Christianity March 25 was seen as the date of the Crucifixion and sometimes as well as the date of the Resurrection. This can be seen in various early Christian authors such as Tertullian of Carthage, Augustine of Hippo Hippolytus of Rome and Sextus Julius Afircanus. Here are some quotes I’ve found:
And the suffering of this “extermination” was perfected within the times of the lxx hebdomads, under Tiberius Caesar, in the consulate of Rubellius Geminus and Fufius Geminus, in the month of March, at the times of the passover, on the eighth day before the calends of April, on the first day of unleavened bread, on which they slew the lamb at even, just as had been enjoined by Moses.- Tertullian, Adversus Judaeos 8:18
Now Christ died when the Gemini were consuls, on the eighth day before the kalends of April. He rose the third day, as the apostles have proved by the evidence of their own senses.- Augustine of Hippo, City of God, book 18, chapter 54
He suffered in the thirty-third year, March 25th, Friday, the eighteenth year of Tiberius Caesar, while Rufus and Roubellion were Consuls.- Hippolytus of Rome, Commentary on Daniel 4.23.3
“And after his burial by Joseph of Arimathea and Nicodemus, he arose from the dead at dawn of the third day after this day of preparation, on the first day of the week, on the 1st day of the first Hebrew month of Nisan, which is forever one and the same day as 25 March”-Julius Africanus, Chronographiae T 93c
However, ancient Romans celebrated a festival of Attis called Hileria on March 25 which celebrates the god Attis and his supposed resurrection. The Chronography of 354 AD is the earliest dating of the associated festival of Hilaria to March 25 I could find. However, some scholars say that this festival dates back to the mid 2nd century to the reign of Antoninus Pius. So did early Christians co-opted the date of March 25th as a celebration of the Passion and resurrection of Jesus or is it just a werid coincidence?
This is most likely a pure coincidence, especially given that the last week of the March or preceding week is marked by the presence of spring equinox, a moment of high significance to many cultures since prehistoric times, so the presence of many various holidays in that period are only to be expected. And indeed, the 25th March was closely associated with the part of Hilaria (that were a week-long festival) dedicated to Cybele, the Mother Goddess.
But Easter is a holiday that was derived from the Jewish customs and was closely associated with Passover to the point that in many European languages it shares the name with the latter. Now, the quotations presented in the questions refer not to the Easter, but rather to its original predecessor, i.e. the actual day when the Christ was crucified, what was recorded as having happened on what 25th March (eight days before calendes of April, but only if you cound in both the first and last day).
The actual date of the celebrations was not set until at least second half of 2nd century, as Policrates, bishop of Efessus and Victorius, bishop of Rome were discussing when the holiday should be celebrated. The former suggesting that the Passion of Christ (Paschal triduum) should be celebrated on the same day as Passover (or 14th day of Nisan, set in accordance with a Jewish religious lunar calendar), regardless of the day of the week (this stance was popular among the Christans in Asia Minor), while the latter maintained that Easter should stress Resurrection rather than death and the holiday should be celebrated on Sunday or, to be precise, on the first Sunday after the Passover (what, in turn, was the dominating idea in Alexandrine School of Theology). Conflict, mediated by Iraeneus of Lugdunum, bishop of Vienne, has eventually been resolved in favour of Victorius. This means that Christians could not have celebrated Easter on 25th of March because, regardless of their approach, they established Easter in accordance either with lunar calendar or a specific day of the week, making it a moveable feast.
In addition, around the late 2nd or early 3rd century, the Easter date calculation encountered another problem. Jews living in diaspora changed the method of establishing the date of Passover and it was no longer set to the first full moon after the spring equinox. This was a problem for Christians, because it was possible to have two Easters in one solar year or none at all. Facing this problem, some churches began to introduce their own methods of calculation (most notably in Alexandria, Rome and, generally in the western part of the Empire), while others kept the old system. Considering this difference as a potential threat to the unity of Church, Emperor Constantine urged the bishops at the First Council of Nicaea to decide a single method of setting the Easter date that would be binding for all Christians. After some deliberations, the Council decided to use the method that is used to this day in an Eastern Orthodox Church, i.e. setting the date of Easter to the first Sunday after the first full moon following the vernal equinox.
So, to sum it up, the fact that the date of 25th March are associated with the death of the Jesus and the Hilaria dedicated to Cybele and Attis is generally a coincidence, as early Christians celebrated Easter as a counterpart of Jewish Passover, down to the date of the latter, meaning that it was a moveable feast from the very beginning. The later discussion concerning the date of the Easter also referred only to Passover and the calculations of the solar-lunar cycle not connected to any existing Roman holidays.
Kelly, J.N.D., Early Christian doctrines, Continuum, London 2000.
Cantalamessa, R., Quiqley, J.A., Easter in the Early Church: An Anthology of Jewish and Early Christian Texts, Lithurgical Press, Collegeville 1993.