I'd start by disagreeing with the premise of your question. The Byzantines were not "repeatedly trounced" by the Seljuk Turks prior to Manzikert. While I certainly wouldn't call the Byzantine defense in the initial stages of their conflict with the Seljuks a "success," I certainly wouldn't call it a thrashing either. On multiple occasions, such as the initial siege of Manzikert in 1054 and Emperor Romanos IV's campaigns of 1068 and 1069, the Byzantines were able to confront and drive the Turks back out of Anatolia. Thus, the question is less "why did the Byzantines suddenly match the Turks" and more "why were the Byzantines suddenly impotent throughout the 1070s?"

To answer this question, we must go back to the reign of Emperor Basil II "the Bulgarslayer." While Basil's campaigns in Bulgaria and the East managed to reconquer vast swaths of land for the resurgent Byzantine Empire, his death ultimately left two glaring weaknesses in the Empire that would eventually lead to the defeat at Manzikert. First, he failed to produce an heir to the throne. After the death of his (childless and incompetent) brother and co-Emperor Constantine VIII, the Byzantine throne was passed around to a series of bureaucrats and generals who each had little personal legitimacy to claim for their rules. As a result of their precarious position, these Emperors had to exhaust the Imperial treasury repeatedly in order to bribe the various organs of the state into accepting their rule.

This problem compounded with the second weakness left by Basil II - an internal political balance of power in the Empire that could only be tamed by a Ruler as strong as Basil II. As soon as Basil II was in the grave and the Empire in the hands if weak Emperors, the Church and the landowning nobility ran roughshod over the central government. The Imperial government found straining to collect the taxes necessary to govern and defend the expanded Empire in the face of a powerful clergy and nobility that were more interested in their own fortunes that that of the State. The biggest loser of this financial crisis was the army, which was allowed to decay from to neglect in the decades before Manzikert. Instead of investing for the long term in the local soldiery, the Empire began to rely on mercenaries who could be recruited on the spot to solve any problems that required martial attention. But between these two factors, the Empire was nearly broke going into the 1050s.

In that decade, the proverbial shit hit the fan when the Empire was faced with 3 external threats almost simultaneously. The first was the Pecheneg Turks, a nomadic group from the Pontic Steppe that attacked down from modern Ukraine into the Balkans. The Pechenegs used many of the same raiding and hit-and-run tactics that the Seljuks would later use, and similarly bedeviled a Byzantine army more used to opposing the conventional forces of the Muslim world. The Byzantines concentrated their remaining resources into stopping the Pecheneg raiders, ultimately finding success through a combination of fortification and the payment of massive sums of gold tribute.

The second threat came from the Norman Franks in Italy. Not only did the new Norman Italian kingdom drive the last remaining Byzantine possessions from the peninsula, but unreliable Norman mercenaries became a mainstay of the Byzantine army. These mercenaries had a habit of rebelling against their employers, pillaging the Byzantine countryside, and on at least one occasion even attempting to usurp the throne itself. Again, large sums of gold the Empire could not afford to lose were spent to pay off and defeat these mercenaries.

The final threat to the Empire was, of course, the Seljuk Turks. Being a nomadic confederation of tribes, the Byzantines found fighting the Turks to be exceptionally difficult. Even when the Byzantines were able to confront and defeat one band of raiders, there were many more coming from beyond the Mountains to raid the Anatolian plateau. Even through all these problems, the Byzantines were able to deter the Turks from settling in Byzantine Territory until the fateful battle of Manzikert. Leading up to the Battle, the Emperor Romanos IV had conglomerated essentially all the free troops the Empire could muster or afford to hire, an army 40,000 men strong. The defeat at Manzikert was less of a result of superior Turkish forces and more the result of poor Byzantine execution - a lack of scouting and communication caused half the army to never engage the Turks in battle, and the other half (including the Emperor) to allow itself to be surprised, surrounded, and destroyed.

There were two decisive outcomes from this defeat. First, many thousands of irreplaceable troops were lost in battle. But more importantly, the capture of the Emperor led to the first of several bloody civil wars and rebellions that occurred simultaneously with the advance of the Turks. Subsequent Emperors prioritized defeating these internal foes over the advancing Seljuks - even choosing to cede territory to the advancing Turks in trade for mercenary services against rival claimants to the throne. While this decision would come back to bite the Empire, from the perspective of the Emperors on the spot this decision was rational. The Turks had little interest in deposing the Emperor or conquering the entirety of the Empire, while the rebels had every intent to decapitate the state and install themselves instead. Low on local forces and gold, the only trade good the Empire had to pay for mercenaries was the land that would just end up decimated by Turkish raids anyways.

Emperor Alexios I's "turnaround" was more a result of being able to focus on regaining land from the Seljuks as opposed to the other threats the Empire faced. After Alexios I was able to bring internal stability back to the Empire, and then defeat the invading Normans and Pechenegs in turn, the Empire was finally able to refocus its efforts on retaking land from the Turks in Anatolia. The result of this was the Komnenian restoration of the 12th century, where the Empire was finally able to dedicate the lion's share of its attention and resources to pushing the Turks back.

Sources:

Streams of Gold, Rivers of Blood by Anthony Kaldellis

A History of the Byzantine State and Society by Warren Treadgold

Byzantium and Its Army 284-1081 by Warren Treadgold

The Byzantine Commonwealth: Eastern Europe, 500-1453 by Dimitri Obolensky