Any battery when being charged undergoes a chemical reaction, part of which involves the generation of gas. In olde worlde acid batteries, you had to constantly check and top up. The modern sealed ones have some tech trickery to allow the gas to be re-absorbed (might or might not be true - theres nothing I can do about it in mine because its sealed, so I choose to believe the "magic" explanation) which might not work if the battery being charged is not upright.
Just something I read - if somebody has one that works OK on its side, it could have been the maufacturer being overly cautious when writing the instruction book. A member of my club had a puffer which the builder had resined in place, and in that case, it would only charge if stood on one end, in the manner of an Atlas ready for take off.