Video frames are stored in a queue that keeps growing until the main thread ticks again.
Can also be reproduced by playing video then clicking the import file button and leaving the file import dialog open.
1) Start playback of video in Media Player window of editor
2) With the mouse start moving the window but do not let go of the mouse button
Audio keeps playing but video stops
Memory starts to grow at about 50-100mb/sec (decompressed video frames)
Memory stops growing if you let the editor resume by letting go of mouse.
Likely happens any time the main thread is blocked (UI or otherwise such as importing a file).
Sample video: [Link Removed]
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-57873 in the post.