probably from pressure building up during the processing. When the jars are upright and there is proper headspace, a bit of air can be forced out of the jar, then when the jars are removed and they cool, the environment creates a vacuum. With the jars on their side, some product got forced out.
I wouldn't count on the jars being safe for long term shelf stable. Bits of food could still be under the lid seal and can spoil from the outside in. Refrigerate and use those jars first.
Check out the BALL FreshPreserving website, they have some troubleshooting guides and great canning guidelines that are very easy to understand and follow as well as basic approved recipes for those who haven't done a lot of canning.
edit to include canning tip- if you don't have enough jars to fill up the bottom of the canning pot, use some glass jars filled with hot water (no lids....) as fillers to keep all the jars upright. The empty jar trick can be used for BWB or pressure canning. For BWB, they will be filled up all the way as the water level should be 1" over the top of the jars. For pressure canning, they only need to be 1/2 full or so.