Here's how to quickly switch between compose and HTML view mode in Blogger post editor:

1. Click the view mode button in the formatting toolbar.

2. Press C for compose or H for HTML view mode, then press Enter.

Blogger HTML/Compose view mode.

Blogger View Mode Randomly Switching

The view mode sometimes randomly switches between the two. From what I can tell, it's just Blogger trying to restore the last view mode you were in. Unless you've made some changes and updated the post, it will keep restoring the last view mode.

This also happens sometimes in the past when I had broken or unrecognized HTML tags, though I'm not entirely sure.