8/10 -- 7:04 AM PT -- Ticketmaster tells TMZ ... "The direction we got from the promoter AEG was that certain seats had views impacted by the production (which they realized during June shows) so they were moving impacted fans. While we help communicate these changes, the decisions are made by the promoter."

Some of Taylor Swift's fans in London have been bounced from their Wembley Stadium seats ... raising questions in the wake of the thwarted terrorist plot in Vienna.

The concert has been planned for months. The 500,000 people who will be attending the 5 concerts beginning next Thursday thought their seats were secure. But a number of fans have been notified they will not get the seats they thought they were getting ... but instead are being moved to other areas of the stadium.

Fans just got emails from Ticketmaster informing them of the seat changes ... some folks say they've been moved back 40 or 50 seats or bumped to entirely different sections.

Welp, we reached out to Wembley Stadium for clarification ... and, they say it has nothing to do with the Vienna plot or security concerns. Instead, they say it has to do with sightlines -- fans having obstructed views from the seats they originally purchased.

Here's what doesn't make sense ... Taylor has already performed at Wembley, so if there were sightline issues they would have surfaced way earlier. To be fair, there was some controversy when Taylor performed back in June, where some fans complained the lighting rigs and VIP tents blocked their view of the stage. But why wasn't that addressed long before her next series of gigs?

It seems odd Wembley would have only discovered the problem just days ago, when Ticketmaster started sending out emails. BTW, the timing does align with the thwarted terror attack in Vienna.

We reached out to Ticketmaster ... so far, no word back.

One thing we do know ... security will be unbelievably tight for the upcoming concerts.