Most likely you have the Auto Select Layer checked on the Move Tool option. You selected the Layer 5 in the Layers Palette but on the way over to the Brush Tool you touched a Layer 1 object on the canvas. I rarely turn on Auto Select Layer.

One thing that bugs me is if you do a selection of something with squared corners then do a select > modify > expand, the corners get cut off. I keep having to look up the way around it then forget for the next time. Another thing with selection. If you Select All you can't do a select > modify > contract but Image Ready will let you do it.