My guess would be that Mikey just walked into the other dog's space a bit too quickly and since the Westie isn't comfortable with other dogs he reacted the way he always reacts but at that point it was already too late for Mikey to realize his error so his "fight and flight" instinct kicked in. No big deal, since both dogs backed out immediately afterwards and no grudges were held.
Taggart has gotten in similar situations before. He's easily excitable and sometimes (not so much anymore though since I guess he learned over the years) I think he just forgets proper dog etiquette and gets right into another dog's face to do the whole sniffing and greeting thing and unavoidably he has ruffled some feathers over the years with that careless attitude. He's better now though, by far. But like I said he's still very excitable so if he's with a dog who's boundaries are easily overstepped I wouldn't be surprised if it happened again.