My dog (~7 year Lab mix) is mostly well-behaved on leash, though he still has a few triggers that we're working on (mostly harmless, but can seem intimidating to other dogs and/or their owners).
One of these actions is laying down when he sees another dog, paws in front of him so he can quickly jump to his feet. It only seems to happen when the other dog is a bit of a distance away but headed in our direction, and doesn't seem to matter the size or breed of the other dog. Unless I intervene or the other dog changes direction, he will always jump to his feet and dash towards the other dog. It's always been in a playful (if intimidatingly energetic) manner, but given the intensity of his focus on the other dog while this is happening, I'm concerned that it may not always be harmless.
I've also seen this behavior in other dogs, sometimes towards my own dog and sometimes with other dogs.