Going on speed dates at the shelter is a very exciting thing, and humping can be expected. Usually there are more issues with rabbits that are outright aggressive towards other rabbits (and I even know of some of those who were deemed unbondable that years later bonded easily to the right rabbit).
I’ve noticed a big decrease in mounting during bonding sessions after a pre-bonding period, where the rabbits can get used to each other’s scents and presence before actually interacting a lot. If you are willing to do the bonding yourself, you could ask the shelter about fostering the bun he seemed to get along with the best (maybe the most submissive one), and then do some pre-bonding with them.
And I agree spring can make a difference! (Assuming you are in the southern hemisphere?)
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.