That's the most likely thing. It's not uncommon during production if the ink hasn't dried enough and a decent amount of pressure gets on the stack of sheets.
It can happen post production, and telling the difference can be difficult.
This stamp (a card proof) was last printed in 1894. The one on the right is a different stamp of the same style bought at the same time and shipped together in the same holder. It didn't have the offset transfer when I bought it, the seller had very nice front and back scans.
So the offset transfer seen here happened in transit between England and the US over a century after the stamps were printed! Whatever the formulation of the ink, it apparently never really dries.
