What happened to Bishan Singh, the son of Dalip Singh, that led to the ascension of Govardhan Chand to the throne?

He died during Dalip Singh's reign.

Answer: He died during Dalip Singh's reign.
Bishan Singh died during the lifetime of his father Dalip Singh, leading to Govardhan Chand ascending to the throne.

Guler appears to have a long tradition of paintings amongst all Pahari schools. There is evidence that artists were working in Haripur–Guler ever since the reign of Dalip Singh (1695–1743) as many of his and his son Bishan Singh’s portraits, dating back to earlier than 1730s, i.e., before the beginning of the Guler–Kangra phase can be found. Bishan Singh died during the lifetime of his father Dalip Singh. So, his younger brother Govardhan Chand ascended to the throne that witnessed a change in painting style.