One of the reasons which is strong at the moment is legitimately fear of losing your job, losing other work, being demoted or at least stuck, and that you're not being judged on vague presence any more, so you're being judged more strongly on what visibly shows remotely.
You think you are doing lower quality work while at home. Maybe you are. Perhaps home has extra distractions, like other people, or your mind can't stay focused as easily as it could at the office. Certainly for those with children at home it's a big problem.
So you feel you need to make up for it by working a bit longer.
Especially because your boss can't see you working.
Maybe you think you are working, but you worry your boss doesn't think so. You have a lingering doubt that maybe your boss thinks you're "clocked in" but you're not really working, you're browsing Facebook or HN or whatever half the day.
Actually, if you had measured it objectively, you spent a surprisingly large amount of time at the office browsing Facebook or HN or whatever before. Your boss did know, but it wasn't a problem, as long as you kept up enough good work to keep things going. As long as the standard set by your coworkers was something you lived up to. The benchmark was set in the office and by your coworkers.
But now, you're wondering if your output still looks good. Because your presence is more doubtful, competitive feelings about productive output and visibility signals sit in the back of your mind.
You're not used to working at home enough yet. So you start to think like an over-conscientious freelancer. Paying attention to your output more. Worked 9-5 with a lunch break, and realised you only "really" did 4 hours good work that day. Feeling like you should probably clock in 8am-7pm to make sure you're seen to be present at least, and maybe you'll end up "really" doing 5 good hours and it will be ok when the performance assessment arrives.
Your boss tells everyone to work normal hours as before. That they don't want burnout or stress.
But you have seen both sides of HR by now; you know friendly platitudes like that are quickly forgotten when the company makes an executive decision to let people go.
Etc.