Monthly Growth - Bigquery
and if you are calculating monthly growth, why aren't you calculating the number of trips in eahc month :
SELECT 
    EXTRACT(YEAR FROM DATE (stoptime)) AS YEAR
    , EXTRACT(MONTH FROM DATE (stoptime)) AS MONTH
    , COUNT(bikeid) AS Trips
    , ROUND(100* (COUNT(bikeid)- LAG(COUNT(bikeid),1,0) OVER (PARTITION BY 'Year','Month' ORDER BY 'Year', 'Month' desc))
                 /lag(COUNT(bikeid),1) OVER (PARTITION BY 'Year','Month' ORDER BY 'Year', 'Month' desc)) ||'%' AS growth
FROM `bigquery-public-data.new_york_citibike.citibike_trips`
WHERE end_station_name LIKE '%University%'
GROUP BY Year, Month
ORDER BY Year DESC, Month DESC