In 1613 Hanyakrakusuma  rose to the throne of Mataram. He began the eastward conquest by an incursion towards Surabaya's southern flank, the Eastern Salient, Malang, and possibly Pasuruan in 1614. Surabayan forces attacked this Mataram army during its homeward march, but was defeated. In 1615 Agung conquered Wirasaba, personally leading the troops there. Surabaya did not commit its troops to help Wirasaba, due to the fear that its other ally, Tuban, would take advantage, betray Surabaya, and attack it from the rear. The conquest of the strategically important Wirasaba posed such a clear threat to Surabaya and other eastern states that the alliance rallied. They mobilized their troops and marched towards Pajang, a city under Mataram's control but ostensibly on the verge of rebellion. However, a Mataram spy in Tuban deceived the allied forces into taking a bad route towards Pajang. As a result, the allied army found itself isolated in Siwalan, near Pajang. This army was surrounded by Sultan Agung and defeated in January 1616. Agung then won victories in Lasem  and Pasuruan . In 1617 Pajang finally rebelled against Mataram but was defeated, and the lord of Pajang fled to Surabaya. In 1619 Agung conquered Tuban, one of the strongest members of Surabaya's alliance. This conquest put Agung in control of Tuban's shipbuilding activities, and therefore allowed him to build a navy to challenge Surabaya's previous naval supremacy.
Question: Why were Surabayan troops defeated?
Answer: Mataram spy