Poor data distribution perhaps? How have you configured your forwarders to spread the data over your indexers? Also: how much data is on each indexer for the relevant indexes that those searches are hitting? If for some reason most/all data is flowing to IDX1, that could easily explain why that indexer is much more busy.
You've also made sure that both indexers are search peers of the search head and search head is successfully able to connect to both for running searches?