Parece que é um recurso chamado Smart Cursoring . Detalhes de aqui :
When smart cursoring is turned on, you can press the LEFT ARROW, RIGHT ARROW, UP ARROW, or DOWN ARROW key to use the cursor on the page currently in view, not at its position prior to scrolling, as long as no content is selected at the original position. To turn on smart cursoring, click Options on the Tools menu and then, on the Edit tab, select the Use smart cursoring check box.
Portanto, no seu caso, clique em Opções no menu Arquivo e, na guia Avançado , desmarque > strong> a caixa de seleção Usar cursores inteligentes .