Not sure what would have caused it, my main advice would be either:
1) Stop the stronger arm at the same reps as your weaker arm.
2) Do an extra set or two for the weaker arm occasionally to help bring it up to par
James 1:16-17 ESV
Do not be deceived, my beloved brothers. Every good gift and every perfect gift is from above, coming down from the Father of lights
With God's help...Mens sana in corpore sano