How is it escalation geza didn't sent like 1000 troops he sent a large contingent to support the Serbs
AFAIK we don't really know how many Hungarian units were sent to aid the Serbs, however it is known that mostly foreign auxiliary forces (Khalyzes, Pechenegs) were sent, and that the bulk of the Royal Army was not present, since it was campaigning in Halych at the time.
in any sense this is an act of war also relationship between the two were already strained prior to 1151, they had fought a war only a generation before
It is reasonable to argue that Manuel was justified in his punitive expedition against Hungary, I don't refute that, however it was still a conscious continuation/escalation of the conflict considering that Serbia was already subjugated by then. In retrospect, the war didn't bring any benefit.
relationship between the two were already strained prior to 1151, they had fought a war only a generation before
The war (1127-'28) between Stephen II and John II was fought specifically because the latter sheltered Prince Álmos (who constantly conspired to dethrone his nephew) and refused to hand him over (or keep him somewhere far away).
It was the son of this very same Prince Álmos who latter succeeded Stephen II, namely Béla II. During Béla II's reign, there was no confrontation between Hungary and Byzantium, the relations weren't strained.
but the last confrontation was because Hungary had broken the agreement still the sole goal of vassalizing Hungary doesn't seem right when Manuel mostly reconquered old byzantine Territory in the border like Sirmium
Vassalisation efforts completely failed because of the extreme unpopularity of both Ladislaus II and Stephen IV, that's why Manuel scaled back his ambitions to just the acquisition of Syrmium and (through Béla-Alexios' claim) Croatia-Dalmatia.
On another note, calling Sirmium old Byzantine territory doesn't really ring that good, considering that Byzantium only possessed the town between 1018 and 1071 in recent history. Prior to that it was in Hungarian, Bulgarian, Frankish, etc. hands. The area wasn't in Roman hands since the 5th century.
Still, it's true that the last conflict was started by the Hungarians. They tried to reconquer the lost territories, but if there are no lost territories to speak of, then that conflict would also be averted.